The B class where originally black with blue boiler, cab and cylinder sides. The buffer beams where painted red and the tenders sides had a red lining. B number 1220, today located in Holland is restored in this condition(but still with enclosed cab and electrical headlights):
https://upload.wikimedia.org/wikiped...220_(1914).JPG
This blue color was not from paint, but a chemical process in Swedish called "blånering" (translating to "blueing") and the result are sometimes referred to as "ryssplåt" ("Russian sheet metal"). So the steel is in some way oxidized (using sodium sulfate I think) and results in this blue color.
But after years of service the blue color darkens by soot and oil. Eventually the locos where painted black (Except for the buffer beams, they where still red), probably around the early 30's depending on when the loco was built. So locos at the time of WW1 would probably have this blue livery still.
But you are right the locos where painted (or if it is due to the conservation process) all black when they were put into storage. But I think the reason the locos are all black in the game is just to make them a bit more anonymous and look less Swedish.
