This costume from The Phantom of the Opera is far and away the most popular costume worn by Christine Daae. There are numerous recreations of the dress now. Perhaps it is due to the heart-rending song the character sings wearing this costume; «Wishing You Were Somehow here Again». Perhaps it's the simple prettiness. My original intent was to recreate the Australian version of the gown as it is rather unique in many aspects. Of course all productions have different spins of the treatments of the designs, and they do change over the years. However the changes ot the Australian version made it more appealing to me as it reminded me more of the fashions actually worn.
The bodice is much longer, which is more in keeping with the 1881 setting. The pleats are much finer and the neckline is more appropriate for day wear.
There is less girlishness about the costume, and more serious student of music as well with the reduction of the amount of trim.
Having decided that these elements are more favourable to myself, I began though to wonder if I should perhaps push it a little further and construct the gown using 19thC cutting and draping techniques.
I did draft the bodice though, and used the base pattern for the other costume anyway.
I have had to break up my page into several sections, please follow them to learn more about the original gown and my recreation.
Main page: photos of the dress in progess and eventually gallery of the dress once complete.
Construction: Each element of the dress broken down to show variations
I have also drafted the pattern for the waterfall drapes.
At first glance the Blue Dress in general looks like a typical bustle dress. But when you start to take it feature by feature, you begin to realise that there is no dress existing that combines all the features used.
The neckline of the original dress was really very low, and scooped with false revers (with a curved neckline these "revers" would have been impossible to simply fold back.) So the change to a high v neckline in the Australian version does bring the day dress more into day dress style.
The use of an apron drape with a bustle drape is fairly common in the 1870s and 1880s, but not combined with panniers.
The bow on the back of the dress is also a very uncommon feature, and really rather hard to find.
The use of stripes and the use of blues, and the use of blue prints is extremely easy to find. Blue floral stripes however are a different matter.
The waterdall drapery is also unusual in the placement and creation of the drapery.
There are also aspects of the shaping of the dress that could come from the first and the second bustle periods, as well as the natural form era the musical is set in.
Maria Bjornson made great use of the book "Victorian Fashions and Costumes from Harper's Bazaar 1867-1898" and used two plates directly, with very minor changes. Compare these:
           
Josefine has pointed out the first two examples in her site for Phantom of the Opera. I strongly suspect the last plate helped in the creation of Carlotta's Black and White dress. Specifically in the use of fur, and vandyked bodice hem and sleeve ends. Many other details do vary, so I think it was simply one source of inspiration.
So this was my first book to look through for ideas of inspiration.
With luck there is indeed an ensemble that shows one of the features: panniers and apron drape:

The sleeves are also half length with pleated ruffles over a ruffle of lace.The dress also has a scrolling floral design. For the Australian version the underskirt is also pleated but otherwise left plain.
A few other plates show some interesting features:

The first image shows a deep curved neckline for outdoors (though decidedly not for winter wear, and it is partially filled in.)
The second has a deep v-neckline and the sleeves end at the elbow with neat and slightly full pleats.
The third again shows a deep neckline (for evening wear however) and rather full short ruffle, but inportantly the elbow length sleeves are caught up on the inside of the arm. This is seen in the Australian Wedding Dress as well as Dressing Gown.
The last shows elbow length sleeves again with an exuberant little ruffle of lace at the elbow. Harper's Bazaar calls this feature "sabot frills of lace."
There were also a few instances of bows at the back of the waist of some dresses in the 1869-71 era, but I dedided not to dela with the, as the position of them was rather different, and the style of the bodices very different (ended at the waist.) However they were an indication of bows being worn.
Nothing else really called out to me, so I decided to look into another source that Maria would have had easy access to, and that is the costume collection of the Victora and Albert Museum. Two items in particular stood out as possible inspirations:

The first simply for the shape, the polonaise is probably a closer approximation to the shape of the pannier drapes of the Blue Dress than any panniers actually seen in fashion plates and extant garments.
The second dress, shows several features in comon with the Blue Dress, firstly it is blue and white striped. It is also trimmed in a darker blue, and has an apron drape. The bodice also buttons up the front and the bodice front ends in an upside down V.
The following sources were probably available to maria as well, being reprints of Victorian fashion plates.
In Full Colour Victorian Fashions 1870-1893, there are a few plates of interest:

The first for obvious reasons: a blue and white floral fabric, cuirasse bodice, apron drape and even a loose bow on the bustle.
The second plate again shows blue and white as a colour cobination, and both outfits have elbow length sleeves with pleatings and frills. The blue and white striped ones beign the most useful as they are double layers lace and fabric.
The third plate shows the first instance of waterfall drapery that is even close to the drapery used in Phantom. However it is reversed, and this type of reversed pleating can also be seen on later 1880s bustle at the side. This plate however shows clearly how such drapery can be created.
The fourth plate shows a low scooped neckline very similar to the Harper's Bazaar one above.
These fashion plates were probably in the Metropolitan Museum of Art, and Maria is known to have used the Met. Opera photos for inspiration for the Hannibal costumes. She may well have visited the fashion plates in the Met. Museum as well.
Then I found Metropolitan Fashions of the 1880s from the 1885 Butterick Catalogue. What a wealth of information!
Firstly many bodices with a false vest front were to be found:

Many were able to be seen with accompanying apron and bustle drape patterns. The apron drapery however often had vertical pleats of gathers as well as the gathers to the back of the skirt. And in a stroke of luck for my Australian version, many show a very long round bodice. However most bodices have pleatings over the bustle, that is until I found this one:

This however was not the last of the finds in the book.

Here we have a neckline so very similar to the Australian neckline, minus the revers. We also have buttons up the front, an apron drape, and a great big huge bow. This bow is perhaps larger than even some of the other international Blue Dresses, but it does indicate a big bow could be worn. There are also vertical stripes on the apron drape, and the sleeves end in ruffles.
The second image shows the best example of a waterfall drape I have yet found, even if it is headed by a butterfly drape. The waterfall effect is clearly created by pleating. The polonaise is also represented in this illustration in a floral fabric. In fact there as some photos of the backs of the Wedding Dress that are clearly very similar to this image.
For images I really am not sure of Maria actually being able to view, but nevertheless show some hint of features in the dress, there are places online. And in fact the interpreters of her designs may well use some of these images in newer costumes anyway.
So far there are a few images to be found:
Costumer's Manifesto timeline of images shows numerous examples of blue in use, as well as ruffles on sleeves (you can see that the fuller deeper ones are more from the beginning of the 1870s while the neater more closer pleated examples are from the latter part of the 1870s.)

The first image shows a nice chevron tiered effect of the back drapery, and diagonal stripes on the sleeves of the green toned dress. The blue and white toned dress also shows a nice large bow on the bustle.
The second image shows a very nice waterfall effect, though it is made from tiers of pelated fabric rather than one continuous piece.
The third image shows more use of strips, as well as v shaped necklines worn outdoors.
The final image shows how stripes could be used to create decorative effects on the bodice, as well as v neckline and a fake vest front.
Truly Victorian is a pattern company that gets rave reviews from practically everyone who uses them.
Hether (who owns the company) has also put online some very nice fashion plates, two of which show very useful details for the Blue Dress:

Because the pages are nice and light in terms of being able to find the images, I've only selected the most relevant aspects of each image.
The first is of the polonaise bodiced gown, which is from 1872. The neckline, shape of the polonaise drapes and use of wide stripes, as well as the exuberant sleeve ruffles all help support the idea that the Blue Dress was a mix of first and second bustle era designs. The shape of the fabric flounce at the elbow is also interesting, and could have been used in a contrasting fabric as is used for the Blue Dress.
The second image is of the train of an 1877 reception dress, which shows the waterfall draping on a train. Again it's not quite the same, but the evidence increases for the drapery to have been possible for a young woman to know about to ask for it to be used on her dress.