Why do SJWs now refer to black people as 'Black?' I realize they're trying to redefine black identity in some way, but I don't get what they're aiming for.
It comes out of the Black nationalist movement. The anti-integrationist wing of the civil rights movement started to reject the newly-popularised term "African-American" on the grounds that black people had never been participants of America, merely residents within its borders, and should not desire to become participants in America until such time as white America could demonstrate the tangible benefits of such a status, which they were sceptical was likely or even possible. At the same time, the growing Marxist influence on the movement lead to a wariness towards "African" identity, which smacked of a bourgeois romanticism detached from the realities of black life.
As a result, they began to develop a concept of black people in America as a group which were not simply victims of economic and cultural dispossession, but which had come to be defined by it. "Black" was therefore not simply a descriptor, but could become an identity in its own right: on the one hand, the lack of explicit national or cultural associations expressed the historical dispossession of black people in America, on the other, the unapologetic identification with the physical features that marked them out to white America represented the self-possession and confidence which they aspired to impart on black people. "Black" became a proper noun, and thus like other proper nouns, merited captialisation.
The language and symbolism of black nationalist remained powerful in black radical circles, even when the politics themselves were largely abandoned in favour of a greater emphasis on community activism. The increasing admission of black people in to higher education brought this language in campus politics, and the recent prevalence of "Intersectional" politics has meant that it gets lumped in, along with a dozen other vocabularies, some of them quite contradictory, are synthesised into a single unwieldy mega-jargon. As a result, you find decontextualised scraps of that vocabulary turning up in places you wouldn't expected, written by people who probably don't understand it, but who know that it is the Correct way to write, and humans being humans, that's usually considered more important.
The tl;dr is, black radicals have been calling themselves "Black" for decades, and the campus-slash-internet left treat radical vocabulary like magpies treat shiny rubbish.