There's actually a case to be made that black Americans would have been freed sooner if the Colonies had never won the Revolutionary War, since Britain ended up outlawing slavery before the US did.

(... but that's historical fiction speculation; there's also a case to be made that but for the pressure put upon Britain by the colonies slipping through her fingers, she'd have insufficient pressure put upon her to outlaw it... Especially if she had one of her largest colonies declaring loudly that a full have of its economy necessitated the practice).