I know several Blackfoot and they insist that the plural of Blackfoot is Blackfoot. I have seen the common english plural of Blackfoot used on maps, signs leading up to the res etc... but if tribal members themselves use Blackfoot as the plural than I can only surmise that it is indeed Blackfoot.
There are many nouns that both singular and plural are the same word. Moose, deer, aircraft are the first ones that pop into my head.