Blacks overwhelmingly voted for it. 70% voted for it. Only 53% of Latinos voted for it
Why stop at just latinos? According to the exit poll data that you're using:
49% of whites voted for it, despite California being a liberal stronghold.
49% of asians voted for it as well.
Those considered as "other" voted for it too, at 51%.
(Analysis of California Proposition 8 Exit Poll Data)
==
Then you have consider the demographics of California itself (2010 census):
57.6% white. 37.6% latino. 13% asian. 6.2% black. 1% native. 4.9% two or more races.
Blacks are outnumbers by practically almost every group in Cali except for two, and by a notable margin.
Would more black support result in prop 8 failing? No guarantees:
Proposition 8
[1]
Choice Votes %
Yes =
7,001,084 52.24%
No = 6,401,482 47.76%
Prop 8 passed by over half a million votes. That is a significant margin. Even if the black vote were to split on this issue, there are no guarantees that Prop 8 would've failed, being that the black population is so low and you'd still have to take into account the votes of other groups, including the big 3 in the whites, latinos, and asians.
Lastly, that 70% figure was from an exit poll. The National Gay and Lesbian Task Force did additional research on the black vote concerning prop 8 and concluded that actually 58% of black voters actually voted for it,
not the 70% figure that was thrown out there at first.
(Black support for Prop. 8 called exaggeration)
==
Thus, the real question those who were upset about prop 8 passing should've been why so many people from the biggest demographics of California voted for it in the first place? More white support and it fails. More latino support and it fails. More asian support and it fails.
Blaming the failure of prop 8 on black folks was (and still is) an easy form of scapegoating to avoid talking about the fact that America, across the board, is much more homophobic than many people want to believe.