Before you possibly come up with a rather stupid argument, maybe you should really first think about the psychological side of all this.
The first question is: Why is this person claiming this, what is his personal motivation? A person claiming something like this is usually trying to boost his own feeling of self-worth by bashing others. He's probably not very confident of his own intelligence, so he tells himself, hey, at least I'm still more intelligent than all those black guys! Of course, that's just something he likes to read into the statistics, it could very well be that he is in fact more stupid than 99% of all blacks,
But the other question is: Why do you even care? Why do feel this strong need to debunk him, even though you don't even know how? I think the most mature reaction would be to simply not react and instead understand that this is a poor guy who must boost his own self-worth by bashing others. You seem to feel personally hurt, which is ridiculous.
Now for the actual topic. The problem is: Statistically, there is a certain truth to what he says. In intelligence tests, blacks tend to perform slightly under-average. Jews on the other hand for example tend to perform slightly above average. But those are just statistical average numbers with absolutely no informative value for individual cases.
In practice, the main reason why some groups tend to perform slightly above or below average isn't because they were genetically born more or less intelligent, but because of social class etc. On statistical average, jews for example are somewhat more wealthy and simply receive better education. Many black families on the other hand are so poor that they simply cannot afford better education.
So it is actually correct that statistically, blacks perform slightly under-average in intelligence tests. But a person who seems to be taking satisfaction at this is actually just feeling happy about still ongoing effects of century-long discrimination against black people.
