The word "slut" carries slightly different definitions depending on which dictionary is consulted. One defines "slut" as: "A woman who has many casual sexual partners". Another defines "slut" as: "A woman who has sexual relationships with a lot of men without any emotional involvement". All definitions make use of words like: "woman", "sex", "many men", "casual". There are a number of words with similar meaning: "tart", and "slapper" (England), to list a couple.
In the 1940's thru the 1960's, the expression "She is EASY" was very popular. Simply put, it meant a woman who was prone to readily allow different men to get between her legs and fuck her. My own wife was always "EASY", especially when Black men were around.
"BBC" of course refers to a Black man with a big cock. But it also implies the ability of Black men to give women, primarily white women, numerous, intense orgasms, and that white women who enjoy the pleasures of such men are "black only".
Put them all together and a "BBC SLUT" is a white woman who exclusively enjoys having sex with a number of Black men for the sole purpose of achieving sexual pleasure (orgasms). My wife is a classic example of a BBC SLUT: she has been "black only" since Middle School, she has welcomed the cocks of approx. 130+ black men (combined total as an unmarried and a married BBC slut), on hundreds of occasions. Her favorite expressions are "For black men, I fuck on the first date" and "So many Black men, so little time".