The illustration does show it as blue, and I suspect it was inspired by the most famous photo of him. Which is actually a series of them in his, flamboyant, full dress uniform:
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The uniform plainly has Confederate rank insignia, and the aiguilette, his French Mande kepi, and sash survive, in the former Museum of the Confederacy.
However, while it may appear "blue" his uniform in reality being that color is
highly unlikely. I would suppose it was of imported blue-gray kersey cloth from Britain. It was plentiful in Texas, where he commanded later in the war, and the uniform coat of his aide-de-camp also survives in the same museum, made of that cloth as I recall. Along with more than few officer's uniforms from the area of the same extant.
It looks blue to many folks in a certain light, and in black and white photography, guess what…
For more information and to get an idea of that color "wool" here you go:
I have been studying Confederate uniforms since 1986. During that time, I have probably seen more Confederate uniforms than anyone on the planet today: the result of making this research my...
adolphusconfederateuniforms.com