Sort of the push against the Mormons is...
1) Holding them to their claims of "tolerance" of still supporting gay couples so that they have certain legal rights.
2) Mormons were responsible for about a quarter of the money raised to pass Prop 8, and most of it came from the Mormon Church.
As for why they don't single out Blacks (who probably gave little money for the cause), there are pretty clear racial overtones (whereas Mormons aren't exclusively White and probably voted for the Proposition in just as strong of numbers), but as only 10% of California's electorate, Proposition 8 would have still passed by a little bit anyway (around 50.3% to 49.7%).
Put another way, there's reasonable grounds to say Blacks did not tip the vote by themselves (as they probably didn't give much money to support it), but Mormons *may* have tipped the scales a little through voting but primarily through money. And frankly, people tend to give a pass to other groups on few issues (gay rights activists tend to be liberal, as do Blacks) and instead focus on those they have much more comprehensive disagreements with (Mormons tend to be conservative). It's more cost effective to wear down someone who opposes you on everything, since you have nothing to lose by alienating them (they were already voting against most of your causes), than someone who sometimes agrees with you (because you might alienate someone who supports some of your causes).
It's a little cynical, but it's also worth noting that those who are trying lure gay rights activists (including talking heads on Fox News and talk radio) against Blacks tend to be unsympathetic to both groups, or at least to the ideology (liberal) that binds them. Just about everyone has blood on their hands.
Also, age is a relevant factor. Only 36% of the youngest group (18-24) supported the bill, but 61% of the old farts (65+) did. Fortunately this problem takes care of itself over time (tolerant adults join the ranks, intolerant old farts die off).
