It is. It's a scapegoat. It perpetuates the hypersexual black man stereotype. Does it happen? Probably. Is it the sole reason or main reason for why it spreads among women? Not even close.
It isn't just black men, I understand "down low" is a slang, but there are white dudes (and all race really) who fukk men on the down low too.
I would say it is the main reason women get it (in 1st world countries). Some get it from contaminated needles, but two dykes grinding and going down on each other aint gonna catch it that way. They need a positive dude to have sex with them.