There was slavery there during the Spanish period and earlier years of the Mexican period. And some pro-Native American types consider the Spanish mission treatment of the Indians to be slavery (even if they worked less than half the days in the year, I think). Did the US inherit the guilt for the earlier slavery by the Treaty of Guadalupe Hidalgo?
Slavery wasn’t even a thing until 1655 with Johnson v Parker. Oh. And Johnson was black so the person who helped codify slavery was a black man.