There may be selection bias in play. That is, perhaps you find more BBC than BWC not because black cocks are bigger on average, but because the black men with big cocks are more likely to be available, or these men have more confidence and are thus more often chosen as partners.
There also could be observation bias. It's not necessarily that they are bigger on average, but they are more memorable. It's common that more memorable events stand out. So... visual contrast of dark skin on light skin (and dark on light does tend to look bigger than light on dark), and an excitingly great fuck, could lead someone to think "it was a huuuuge dick!"
(Also is a grower vs. shower element -- people evolved in cooler climes are more likely growers than showers, people evolved in warmer climes are more likely showers than growers... so if after you are accustomed to seeing black cocks that are bigger when flaccid and white cocks that are smaller when flaccid, I can see how that could affect perception of how big they are when hard.)
Not judging observations either way, just noting that even if on average black dicks and white dicks are the same size in the general population, there are reasons why observations might not align.
