Blackfeet
Singular and plural in U.S. context; do not add s for plurals.
Blackfeet in the U.S.; Blackfoot (Siksika Nation) in Canada. In Canada, the confederacy is Blackfoot Confederacy in English, but Siksikaitsitapi in Blackfoot.
The language is Blackfoot.
See also AMERICAN INDIANS, NORTH AND SOUTH.