If they really have a no-black-only policy then that's really ridiculous. But I don't know, I can see where they're coming if they don't want to intermix with any races at all since Indigenous American numbers are dwindling.
On the other hand, I go to the Shinnecock Pow Wow in Long Island every year which hosts tribes from all over the country and even Mexican and Peruvian tribes. Walking around seeing the diversity is beautiful. It kind of reminds me of Latinos. There's people of all colors, hair types, etc. Even the actual Shinnecock has a lot of black/Native mixed people and they're not seen as any less Native.
If you were raised in the culture, no matter what you're mixed with I don't see the problem.
I will say I
every year during the Macy's Day parade because the Native American float is always full of blonde/brunette light eyed white skinned people with the ONE brown skinned person wearing a headdress.