I’m surprised that the black Confederate population is allowed to vote at all. And with how the USA, CSA, and France all interacted here, it would not surprise me if the WW1 analogue became a three-way conflict instead of the traditional two-way war.
The people who won the First Confederate Civil War (the current Confederate political establishment in ITL 1900) are mostly racial pragmatists, guys like Mahone, Longstreet, etc. Guys who OTL were pro-reconstruction not because they were ardent believers in racial equality, but moreso "hey, this is how it works now. Get used to it."
Their racial strategy is to basically not talk much about it, but basically do enough to be the most palatable option in Confederate society for black voters, who are mostly desperately poor, and then get those black voters on board with patronage politics (we built a school in your town, vote for us!) This also fits in with their general interventionist economics. ITL, I'd bet in polite (aka racist) company, they would probably justify this by arguing if they spend all of their effort suppressing 40% of the population, they'd be weak and divided as they were during the Spanish-Confederate War.
Obviously this is only my analysis, but it reminds me a bit of "votebanks" in India or other countries, where marginalized groups vote en masse for a certain candidate or party based on clientistic reasons, while middle-class members of the majority ethnicity view this negatively, either because they find it grossly corrupt or they just don't like the marginalized group in question (or sometimes both?). It's why the ITL "Progressives" are a mix of both hardcore racists and good government types.