Black majority south
The Union follows more of an antislavery policy. The Confederacy retreats into Texas after the Union concentrates all of the Eastern Theatre forces on the Potomac, instead of sending the expedition to the Peninsula. The Confederate army is forced into battle by methodical destruction of slave owner's property as the Union advances, and is destroyed as the Union army advances until confronted and then digs in, which forces the Confederacy to make repeated frontal attacks.
The Union follows a policy of enlisting slaves in the army. There are more black troops. They are used in Pensacola to cut Florida's contact by rail to the rest of the south as an anti blockade runner strategy. As the front advances into the Confederacy there are more and more black troops, till after Louisiana the Union army is half black. The Union army moves into Texas before the Confederacy surrenders. leaving a railroad behind.
The white troops joined earlier and are demobilized earlier, leaving the army with a bigger black majority when they are sent to Texas as the war winds down to threaten Maximillian in Mexico. When Max falls, the black army is demobilized in place, giving Louisiana, Florida, and Texas a black majority.
Eotvos gravity meter work is copied by the Texas government's experiment with surveying resources. The first American dipmeters are operating in 1868 instead of 1924, kicking off the Oil Age a little early by finding Spindletop and making Texas and Louisiana rich because they have lots of salt dome oil. Texas increases it's population rapidly in a boom as the railroad is extended across south of the Rockies instead of through the middle.
Texas is split into five parts as per it's treaty with the US on joining in 1845. The blacks now have 14 senators for the 1876 election and just put the Republican president over the top. The political power this gives them helps cement their place in society. They also get six new Senators for New Mexico, Arizona, and Southern California as the railroad ends in San Diego and increases the population along the Colorado faster in this ATL.
Finally, in 1884 Douglas is the Republican candidate as the two white candidates fight over who should be President and back him as a compromise. Douglass is someone thay hate less than each other.