When I do my 7th Ill mounted Rifles impression I use a 1851 mounted Issue blanket. It is Plain Dark Blue and is 60 x 80 This is a great blanket and the horse loves it as a saddle blanket. I have one I use for the horse one for me and one for the Mrs cuz she thinks its pretty lol. I bought mine from a source outside Gettysburg last year and they are well crafted but expensive compared to sutler prices. Try and research your unit as much as you can to find thier quartermaster issue sheet or a discharge paper that states the uniform and accourtrement issue it will sometimes be specific as to color and such of blanket and other items. Sometimes it will just say Blanket which isnt helpfull. I hope this helps a bit.
Drew Ingram (USMC RET.)
WIA: Operation Iraqi Freedom
"The tree of liberty must be refreshed from time to time with the blood of patriots and tyrants." (Thomas Jefferson)