I assume this means that their sample of adults (or registered voters) included more than 12 percent blacks, which would tend to skew the numbers towards Obama.
This is a bad sample. It isn't representative of the people who will actually vote in this election. In every election since 1988, the percentage of Democratic voters never exceeded the percentage of Republican voters by more than 4 points. In 2004, the Democrats did not have an advantage at all. Here, the spread is a whopping 10 points.
Another problem with this poll is that the likely voter numbers were nearly identical to the registered voter numbers -- yet another indication that the sample was overpopulated with Democrats. According to Gallup, the likely voter pool should be about 4-5 points more Republican than the registered voter pool.
"Show me just what Mohammed brought that was new, and there you will find things only evil and inhuman, such as his command to spread by the sword the faith he preached." - Manuel II Palelologus