Was the total for Blacks 200? If so the oversampling was not that much. Instead of 12% they used 16.67%.
This would suggest that they should have had 1200 x 12% = 144 Blacks represented in the poll.
Instead they had 144 + 200 Blacks - 28.7% of those questioned instead of 12%. That is more than double representation in the poll, compared to the population...