Black Texas
Santa Anna is kicked in the head by a horse, Urribe successfully beats the Texans and liberates the slaves. The Mexican government figures out that Texas is going to be settled by somebody and decides to settle lots of freed slaves there? There were around 250,000 freed slaves in the US and lots more in the Caribbean. The exslaves are certainly motivated to defend the border against more Anglo immigrants. The northern states aren't enthusiastic about fighting a war for slavery and conquering more territory for slave owners.
Without the Mexican cession California is settled by the Swiss or other Europeans to the extent that they outnumber the Americans? So it's sort of like America, but different? British? Would some blacks from Texas go by land? There was gold and other minerals in Colorado along the way.
When the Civil war happens the Union has an ally on the south, important for stopping smuggling of military supplies, and as a base for blockader warships. The Confederacy has to mount an invasion a long way over unsupported land, with no river or sea transport, in the face of the Union navy. They try, but not very successfully. Maybe the Union accepts Texas as a number of black states and they they wind up with a monopoly of most oil in the lower 48?