I can give you a link to a database where you might be able to do some research. http://www.accessgenealogy.com/native/
If you click onto the "Index and Database of Indian Rolls", you can search all of the Indian Rolls. What I would suggest is to try different spellings of the name, it is possible the name was misspelled on the 1925 census; you can even search by just putting the first few letters of the surname in the box, and a list of surnames beginning with them will come up to scroll through. You may also want to check out some of the other databases to see what else may be of help, there are places for queries where maybe someone would be able to tell you more about searching someone of Blackfoot ancestry.
You might want to check out looking for your gggrandfather by trying Oklahoma, that's what the birth location reads in 1925. It would have also been known as Indian Territory in the 1800's, so checking census or anything should be looking for those areas, although I must tell you I don't think there was any census done for either area until abt 1900. There's an interesting bit of info on Indian Territory at Wikpedia which may also be of help, it mentions that the tribes brought their African American slaves with them to the area, and it's possible that both of your ggrandfathers may actually have been a part of the area. http://en.wikipedia.org/wiki/Indian_Territory
Check out the USGenweb site for IT,there's lots of information and links that may be of assistance. http://www.rootsweb.com/~itgenweb/
And again, I would try different spellings of the surnames in all of your searches, it's the kind of surname that can easily be misspelled and who knows what a census taker may have listed it as. Also, a copy of the Death Certificate might show who both parents were, if you haven't done so already, that would be something to send for.
You should check out and learn as much as you can about the Blackfoot Tribes, and I would try a Google search to find that; here are a few I found:http://www.utexas.edu/courses/wilson/ant304/projects/project...http://www.blackfeetnation.com/http://www.accessgenealogy.com/native/tribes/blackfeet/pastp...
Another thing, have you ever been to the Afrigeneas.com website? If you haven't, I'd suggest you also try there, they have an African/Native American Forum where people doing research on black/Indian families may be able to help you. The many people on the forum can give you ideas or tips, possibly tell you where to look, or you may even run across someone searching for the same family. http://www.afrigeneas.com/
once there, click onto the FORUMS tab at the top of the page, and scroll down to the African/Native American Forum. The entire website is geared specifically for African American research of all kinds and there are really a bunch of helpful people on all of the forums to help with various research, so you might also want to try posting to on the the other Forums on the site for Genealogy & History, and the Surname & Family Research lists, too.
I don't know if you've tried these before, but if not, maybe some of these ideas will help you.