The Choctaws were known by the United States as part of a group referred to as the Five Civilized tribes. This was because they lived a way that Europeans could see as having some sort of civilization. Who knows what other tribal people were referred to–but they weren’t seen as having ‘civilization.’
Unfortunately, part of this civilization on the part of the Five Tribes was to adopt certain aspects of European culture. Even more unfortunately, the practice of slavery was adopted by the Choctaws.
Choctaws, then, had Black slaves.
Being a member of the alleged civilized tribes didn’t give a tribe any more protection or good relations than being a “non-civilized” tribe. So, when Andrew Jackson basically felt like it, he enforced the Trail of Tears.
The Choctaws’ Black slaves walked the Trail of Tears with them.
Upon arrival at Indian Territory, the Choctaws freed their slaves. And, today, the Freedmen are full-standing citizens of the Choctaw Nation of Oklahoma.
So don’t assume that every tribal person is brown. Some are indeed Black. And this is how they got to be citizens of a tribe.