If we were to use colonial law as precedent, "America's first black president" is actually White.
According to colonial law the race of a person was determined maternally, not paternally. That is, the person's race was considered to be the same as the mother, not the father.
Children born to White women in colonial times were legally deemed White, even if their fathers were black or Indian.
From that perspective Barak Obama is legally a White male because his mother was White.