I don't see how increasing the number of slaves in France, or the number of free black people in Virginia, will decrease the violence of a revolution. Could you clarify this?
Basically, any outsider group, whether it be blacks, Indians, or even Germans, in the country in large enough percentages will probably tamp down radicalism in any revolution....as any country which has a large foreign population is likely to have powerful intrestest on both sides that will not want things to spin that far out of control.
In the American Revolution, for instance, NC, SC, and Georgia were dead set against enlisting blacks in the army for any reason, and Georgia went as a far to declare that should the Continental Congress try to force it to raise black troops, it'd lay down its arms an surrender to the British immediately. (And SC made similar, if less explicit rumblings)
I don't think that type of mindset is unique to the American situation. I don't see a truly radical revolution transpiring anywhere that has a large body of "foreigners" in its midst.
Not say it's impossible. But anything as radical as the French Revolution in politics before Napoleon would likely meet a wall of economic and social implausibility with any colony below Virginia, with Virginia itself being a stretch although I don't recall any of its great statesmen at the time specifically stating or even implying that they'd surrender than see the local social order disrupted, and they were the southernmost state to offer freedom to slaves in return for fighting the British.