Once white girls fuck black guys in porn, there's this...mark on them now. Their potential value goes down.
That's what the boyfriends and the racist producers want them to think. There are also a lot of porn stars from the south who just can't abide it they'll do a 15 man gangbang with double penetration and get coated in spunk and think nothing of it. But have sex with a black man oh my god never.
You can look at a porn star like Sasha Grey or Belladonna who went against that sort of 'advice' and done what they wanted. Sasha Grey is doing lots of work outside of porn so doing scenes with black guys hasn't harmed her career one bit.
You also some pornstars who think they can drive up their prices for when their career flags then they can move into interracial stuff for a big payday.