Sharon:

I have no doubt that black regulars heard, read, and probably used the term "buffalo soldiers" long before l900. The term had found some use in the national and local press--and it casn be found in a few examples of private (by whites) correspondence.
Of course, there is not a large body of extant materials written by these troops, at least not while they served--the best "original" sources--"Voices of the Buffalo Soldiers"--are almost always found in post-service penision applications and in the scattering of letters they wrote to Winners of the West during the l920s and 30s.
As far as I know, the first KNOWN and IN PRINT use of the term buffalo soldiers by (a then retired) black regular comes in l930.