There was a slogan, "defund the police". What it means is reallocating funds to try to mitigate the factors that cause crime and the violent interactions between police and people of color. But what voters heard was "defund the police." Lots of liberals embraced it...and not surprisingly...that didn't go over too well with voters.
I agree to a point, BLM scared off some white voters.
Trump got 57% of the White vote in 2020. This is actually worse than he did in 2016, when he got 58% of the white vote.
He got 12% of the black vote (up from 8% in 2016) and 32% of the Hispanic vote (up from 28% in 2016). Both of these are trend lines, however, because Romney did better with whites and worse with minorities in 2012 than Trump did in 2016.
So if BLM really was that much of a factor, he would have done better with whites and worse with minorities, not the other way around.
I wouldn't read too much into any of this, however. The fact is, Trump had the power of incumbency. All other things being equal, people will go with the devil they know. Even Jerry Ford, who had a reputation as the bumbling fool with a WIN button who pardoned Nixon, STILL managed to squeak out 48% of the vote in 1976. Trump being a master of media manipulation, managed to dominate the conversation the whole time. Biden is going to have a serious discussion about policies, he had to contend with a bunch of crazy tweets that dominated the news cycle.
Now, on these congressional districts, again, this isn't too unusual. Most of these districts were drawn (gerrymandered) to favor Republicans. That a Democrat took some of them at all was a miracle in and of itself. What amazed me is that they managed to retain some of the ones they did.
For instance, Lauren Underwood managed to pick up Denny Hastert's old district, which is a strong R district in 2018. She was a black woman who managed to win in a district that is 85% white. She barely squeaked by to win a second term over Jim "the Milk Dud" Oberweis, who keeps running for things and losing. If I were a betting man, I wouldn't put much stock in her winning a third term in 2022. But it shows that "all politics are local". There were a whole lot of factors going on that had nothing to do with the national scene.