A previous post referred to Corbyn's statements on anti-Semitism being a posture not a position. That struck a chord with me because it reminded me of my own treatment at the hands of left-wing friends and acquaintances (activists mainly, rather than ordinary 'civilians', for lack of a better description).
I am black, and when I discuss racism with other black people we have never believed there to be no racism on the left. We have tended to agree that the right is more racist, but that there is racism on both right and left albeit of different kinds.
My experience has been that black people are prized as a totem of inclusivity on the left, proving the tolerant liberalism of their white compadres, provided that they conform to the role and the beliefs ascribed to them in the left-wing narrative. (This tends to be every bit as stereotyped and de-personalising as right-wing narratives, albeit not generally negative or vicious.)
The name given to this conformity is usually 'authenticity'. 'Authenticity' is important if you are black or brown, though no one can really explain why very clearly. If you fail to be authentic, you are accused of being self-hating, a race traitor, Uncle Tom/Aunt Sarah, out for yourself at the expense of the collective, a coconut etc.
Inauthenticity is often demonstrated by being any of the following; religious, intellectual, traditional, successful, well-spoken, more socially conservative than your peers, not activist enough.
It is not necessarily accepted that being black gives you insights into, well, being black. You have to occupy a certain space and act out a certain role to be given any such credence.
This is how I ended up having a very privileged white woman tear a strip off me for being selfish and a traitor because I had not gone on an anti-apartheid march, having my experiences of racism dismissed by white people because I am middle-class with educated parents and accordingly, not truly in the know and generally being tediously lectured at by non-blacks with zero experiences of our lives.
And really, this is where a portion of the left (a minority, but perhaps disproportionately comprised of activists and office-holders so appearing to be larger and influential) has been for a very long time, almost certainly in relation to Jews and many other groups as well as black people. The underlying attitudes and beliefs towards these groups are not examined. Many people have thoroughly 'unsound' views, but heavily overlaid with an unshakable 'we're the good guys' narrative and to keep that narrative going, require BME people to play a prescribed role rather than feature as equal, autonomous, individual actors.
Any Jewish person supporting Israel has abandoned their prescribed role. The difference between now and the past described in lisalisa's powerful post being that left-wing attitudes to Israel have shifted from seeing it as a new, modern idealistic state where Socialist ideas could be established to an oppressive European capitalist colony brutalising Palestinians on race grounds.