The myth of "BBC"--and it is a myth--is rooted in American slave culture. It was part of the entire "animalizing" narrative that began with the slave ships and slave auctions in the American South. It continued after the Civil War and during the Jim Crow / segregation days as a way to discourage intermarriage. At that time, a large penis wasn't desirable sexually, so it was actually used to DISCOURAGE white women from being romantically involved with black men. It went hand-in-hand with the "black rapist" myth. The idea was to make black men scary and intimidating.
Now, things have changed... The days of racism and bans on interracial relationships are over, but the "BBC" myth remains. It's obvious why. With a culture that now values large penis size, black men have every incentive in the world to embrace it. Media likes to perpetuate it, as anything sensational is great for show biz. Some have political/social motivations, much of which revolves around the diminishing of white masculinity as retribution for past transgressions, real and invented. So, those with an agenda, who believe that white, heterosexual males represent some sort of great evil, use penis size as a way to chip away at a perceived power structure--and, quite frankly, as revenge.
Whether one acknowledges the current psycho/social motivations of many who currently perpetuate the myth and an agenda that is quite simply hating on white men and seeking revenge, one thing is certain... it is a myth that is proven time and time again by science and aposteriori evidence. The other thing that is certain, is that the myth IS rooted in racism against black men, whether people choose to acknowledge it or not. That is historical fact. Perhaps this last point is why I find it so incredibly bizarre and particularly offensive that European culture has embraced the idea of "BBC." I think most Europeans are ignorant to the American slave origins and are simply jumping on a bandwagon kink, so I try to give the benefit of the doubt.