Indian ancestry of Margaret Emma Lucas 1893 South Dakota

Hi my grandfather Robert Marion James was born in 1893 South Dakota to Joseph Marion James and Margaret Emma Lucas. I can't find any info on their marriage. The family believes Margaret was of Indian descent but we have been unable to find any info on her. We know Robert attended an Indian school in his early years as he told stories of playing baseball against the Blackfeet Indian girls. We have no ideawhat school. Margaret may have died young as Robert was living with his great"James" Uncle back in Kansas by the time he was 17. So he was raised as a white after that and no records where kept by the family of his Indian heritage. He absolutleylooks Indian in his pics. Some family DNA tests do show Indian blood. Any help would be appreciated! Thanks Liz