I always wondered if was a quasi-slur. (At the very least, it seemed like a rude/inappropriate/archaic term.) By contrast, Blackfoot is the name of a specific tribe. It is the name used by the tribe. You could get buy in from actual tribe members. Florida State Seminoles and Utah Utes I think both have relationships with and approval from their namesake tribes. It is way cooler to be specific and have a relationship with the modern members of a tribe. You might make some new Washington football fans in Montana.