It wasn't a militia unit, it was a Volunteer battery that was already in the process of formation before the raid began. Its officers were all Blacks.
For its history:
http://skyways.lib.ks.us/museums/kng/indc.html

For its roster:
http://skyways.lib.ks.us/genweb/archives/statewide/military/civilwar/adjutant/ibat/ibat.html