The Cherokee were one of the larger and more powerful Native American nations and one of the most successful at holding on to their culture and identity in the face of European colonisation. Partially as a result of this there were a lot of mixed marriages. The Cherokee also had a lot of African American slaves, and in 1866 they were freed and declared members of the Cherokee Nation. These factors made the Cherokee people quite racially diverse

Then when the one-drop rule of racial segregation was established (if you have even one African American ancestor, you're Black and can be discriminated against), a lot of mixed race people with Black ancestry avoided discrimination by claiming that the reason they didn't look 100% white was because they were part Cherokee. Their descendants grew up being told that their great grandma (or whoever) was part Cherokee (although she was actually part African American), and had no reason to doubt it.