Nothing physically, suspect it is all psychological.
The BBC is a long established female fantasy, I suspect the contrasting skin tones plays a big part.
But, I’m afraid the roots of the fantasy stem from the 18th Century and have racist/colonialism undertones - the female rape-fantasy of the pure white woman being ravished by savage natives. It is well established now that those conservative ladies of that time, where even the glimpse of an ankle in public was extremely shocking, were extremely kinky behind the facade.
Obviously, not like that now, but read some historic erotic literature and it is certainly a possibility of how it all started.
