I noticed it as a teenager when I watched TV. Anytime there was a redhead in a show, she had a black boyfriend/husband. I pointed it out to my coworkers at the time, and they said that's ridiculous. Then, when we came back to work after our weekend, they'd all come up to me and say, "Remember what you said about redheads the other day? I was watching TV and... I noticed it. You're absolutely right, they are always shown with black guys!"
So I think it's just that women are extremely susceptible to (((propaganda))). Redheaded women irl see it on TV, and it subliminally makes them want to mate with black men. You also see traditionally redheaded characters get race-swapped more than anybody else in all these modern adaptions and reboots.
That all being said, the redheads I see with black guys irl are usually also ridiculously fat.