the thread goes into detail but basically when prop 8 passed a lot of blame was thrown to black liberals who would vote for obama but not for prop 8. turns out pro-prop 8ers hit up poor, mostly black, neighborhoods with robo calls featuring a quote from barack obama saying "I believe marriage is between a man and a woman", cutting out the preceding "I'm against proposition 8 but". they also made the language of the vote confusing.
so it's not specifically black voters fault or here's dm's reply actually:
as you said, they spent a lot of money on deceptively wording this. they also sent out robo-calls with clips of Obama saying "I believe marriage is between a man and a woman" while leaving the "I oppose proposition 8, but" that was supposed to preceed it. when campaigns like this are run, what do you do to counter them? you inform people, and surprise, no on proposition 8 people didn't really reach out to black people (read: go to black neighborhoods). so you have to fault the opposition campaign for that too.
what really makes it a problem is that like 10 percent of the electorate was black. 52 percent voted for it total, 70 percent of black people did. if you subtract that number from the 70 percent, the deviance of the higher black vote on the total vote is somewhere around 2 percent, which doesn't come close to accounting for a 5 point split.
you can analyze and ask why the vote was that much higher, but you can't say that it had a decisive effect on the outcome.