Bright Blue Dye
I think that this colour could only come from an aniline dye. Blue dye is hard to make. Early dyes were made from indigo or woad, but I couldn’t find examples of these that produced that vivid a colour for fabric dye. Bright, vibrant fabric dyes were a product of Victorian era advances in chemistry. The Dreamstress gives a great summary of early chemical dyes and a gorgeous Pinterest page with examples of the new colours. The colour above looks pretty close to Nicholson’s Blue, developed in the 1860’s. Here’s an example:
Interestingly, Aldern Foxglove in Rise of the Runelords has the same colour in his jacket.