Well there's class traitors, wreckers, italians, irish, dixie whites, kulaks, etc to carry out 100-150 million dead. No reason for them to stop with blacks.
Please point out why Italians, Irish, or Southern Whites might be killed when all three groups can easily be won over and basically would be, since quite a few Italians and Irish were on the "left", albeit often enslaved by local political machines which certain Socialist Party groups like the Jews and Finns were not. You do know the Bolsheviks were a minority faction yet seized power regardless and managed to incorporate a lot of former opponents? And that Stalinist-style repression isn't inevitable, especially given the American tradition of democracy (which didn't exist in Russia), which with machine politics we can assume would lead toward a guided democracy (with a puppet Socialist Party and maybe a few puppet local parties led by the Communist Party?). And the South was full of potential support, from Huey Long and other Southern populists to Appalachian miners.
Regarding killing blacks, I already pointed out the reality of what American socialists and communists thought of the matter. Even adding some elements of Southern populism, like Thomas Watson in his later years, to the mix, you don't anything worse than pre-existing segregation, and as I said, you have local politicians who can use blacks as part of their power base like Boss Crump did in Memphis.
Really, try being realistic instead of whitewashing what a communist regime would be like.
I am being realistic, since I said an American communist regime would kill perhaps 5 million people in addition to those killed by communists in the Second American Civil War. I'm not whitewashing anything, since I know the "red" quality of the regime would be akin to the red of human blood, and I know they would very likely promote a lot of policies which wouldn't be ideal, to say the least. But you are doing the exact opposite and giving the idea that 100-150 million might die in a country which didn't even have 150 million people until 1950. That's utterly ridiculous. You clearly haven't examined the conditions of Gilded Age/early 20th century US and the existing socialist/communist movement, as well as the historic nature of the US and the compromises which the revolution will inevitably make. That, IMO, is a good clue as to how a communist revolution will turn out in any given country, barring something like communism being imposed by force as the USSR did on Eastern Europe in the late 40s.
It's all on you to refute my points instead of going by some ridiculous generalisation of communism which fails at explaining OTL communist regimes, let alone a hypothetical American communist regime.