We're just going in circles here. And I'm tired of arguing that America, Britain and France are all busy carrying the idiot ball in a fictional war when the diplomacy doesn't actually hold up to closer inspection.
A) Britain would never be allied with a slave holding state. They'd probably be on friendly terms with the USA
B) America would never be audacious enough to declare war on a CSA aligned with two major powers
C) Britain would make a harsher peace on the USA than the book
We're arguing C, which is kind of pointless because A and B are even more preposterous.
A) HFR says that Britain quite clearly will only support the CSA if they "free" the slaves. The CSA is also the defender, and isn't conducting any offensive action, and supporting a defender was in line with British foreign policy at that time (OTL or 191).
B) Blaine doesn't think he is getting into a war with Britain or France, because he knows they won't support a slaveholding state (as you said above). One of the founding principles of the CSA was slavery, so he has good reason to think they won't just scrap it for foreign support. A CS/US-only war is an easy US victory, which is what Blaine is reasonably expecting.
- BNC