I'm really tired so I may double back tomorrow to this, but in essence historically the American left has fairly consistently abandoned black people in almost uniformed fashion in favor of social democratic reforms with explicit carve outs to exclude black people. The easiest examples to trace are the exclusions carved into the New Deal.

This has lead to a fairly consistent skepticism for the left by black Americans, particularly in cases where it isn't black led. With Bernie's general social democratic policy, the skepticism was already there and it also met a cross roads with white-chauvinism whenever the skepticism was expressed essentially policing the historically earned skepticism with a combination of condescension, distain, and vitrol. Worker's parties and the like have historically seen black exclusion, and many of us are either vaguely or acutely aware of this phenomenon.

The exclusion isn't just throwing black people to the wolves for self benefit, but has also seen actively sabotaging movements at the inclusion of black people. If I recall correctly, there is a noticable drop in support for social welfare policies if they are phrssed in a way where there is active awareness that black people would benefit from it, even if the person being questioned would benefit as much or more.

It's beyond "fuck you I got mine," and veers into "fuck getting mine if you get yours."

Edit: I'd recommend a reading or video but I'm struggling to recall much with my current fatigue. lil bill's The Left has a Whiteness problem covers it however a massive disclaimer needs to be placed since he has little to no regard for MLs (he's from my gathering a black nationalist though does seem to think socialism is needed for black liberation). I also know sections of Revolutionary Suicide by Huey Newton covers how it manifested into organizational issues for the Panthers but I still haven't finished that so can't say to the full degree which it's covered.

For further off the cuff modern context, a decent amount of Bernie Bros deviated to the populist right or are part of spheres such as "Breadtube" where diamonds such as ContraPoints and Destiny were staples. Many people that nominally diverged from them still haven't shaken the chauvinism (though in some cases this seems to be willfully maintained) and frankly don't seem to really care for the concerns of Black Americans further than our use for coalition building for their re-elevation.

This isn't to pretend I think the Black-working class is a day away from revolution either, while I am a bit young from witnessing this, from what I've seen there was mass liberal buy-in during the Obama era, where much of the democratic stronghold before appeared tactical, a lot of it seems to have transformed into legitimate believe in the system.