There have been some pretty blankets. The Whitney or trade blanket came in whites, garnet, blue, green, camel with contrasting stripes. The blankets issued during the CW were red, blue, tan, gray, etc. often with contrasting border stripes. Earth tones certainly. Native american blankets, especially Chief blankets, were extremely colorful. And many very modern looking as far as patterns.
If I remember correctly, Pendleton woolen mills have been in production since our era.
Quite often you can get nice weight wool blankets in drab colors as military surplus from one of the discount catalogers that deal in surplus European military items. Swiss, Italian and others. I've seen tan, gray, dun brown, olive.
Stay away from fleece and the bright, neon colors and I think you would be safe. There are many Sutlers that cover the Trader, Civil War and Indian War era that sell blankets. I think Pendleton still offers a line of blankets that they term from their historical line.