Stephanie,

I believe your ancestor was probably a contractor. The Black units (Buffalo Soldiers) did not deploy to Montana and the Dakotas until the late 1880s.

I checked the Civil War database for William H. Comer and E.L. Comer, and the only name that came close was a Eli Comer who served in the 29th United States Colored Infantry Regiment from Illinois.

I will double check and advise.

Bennie