You should be able to find the information that you seek from the following website
Good Luck in your search.
Dear Ms. Watts-Homen,
Thank you for posting your request on History Hub!
For an overview on how to begin your research, we suggest that you review NARA’s Native American Heritage as well as the FamilySearch Research wiki for Indigenous Peoples of the United States Genealogy.
When embarking on Native American genealogy, please note that the records in the custody of NARA often only detail those living on the reservations or being administered by the Bureau of Indian Affairs. The tribes that collectively are known as the Blackfeet were located further west and north and into Canada. Since the U.S. government did not interact with that tribe until the mid-19th century, and even then did not begin to compile detailed individual records until the 1880s and later, NARA’s records also would reflect that.
We searched the National Archives Catalog and located 180 series referencing the Blackfeet Agency in the Records of the Bureau of Indian Affairs (Record Group 75) that may contain information about your grandfather. For access to this series, please contact the National Archives location via email as referenced in each series description.
Due to the COVID-19 pandemic and pursuant to guidance received from the Office of Management and Budget (OMB), NARA has adjusted its normal operations to balance the need of completing its mission-critical work while also adhering to the recommended social distancing for the safety of NARA staff. As a result of this re-prioritization of activities, you may experience a delay in receiving an initial acknowledgement as well as a substantive response to your reference request from three NARA reference units. We apologize for this inconvenience and appreciate your understanding and patience.
We also suggest that you contact the Blackfeet Nation to inquire about your grandfather and his family.
We hope this is helpful. Best of luck with your family research!