LOL. There's nothing wrong with stating that he was black. It's a fact that statisically you're more likely to be stabbed by a black man than any other race.
Anyone that cries "racist" doesn't have a clue what they are talking about.
It's also a fact that you're more likely to be stabbed by a man rather than a woman, a junkie rather than a doctor, a young person than an old person - but that doesn't mean that it's worth mentioning as if no-one out with those groups could possibly ever commit a similar crime.
It's not 'racist' to mention that he was black - but it does suggest that you think it has more of a bearing on the crime than other factors which can also be linked with a statistically increased likelihood of committing the crime (like being male, young, etc).
The reason I originally queried the 'I'm not racist..' statement, was not because I thought the OP was a racist, but just that bringing attention to the fact that the other guy was 'of a different race' had little bearing on what had happened, as this sort of aggressive behaviour is just too common in general, and not purely a problem of one particular section of society.