It's really funny what you come across when you're digging around the Internet doing research for hot romance or erotica novels and novellas. Yes, that was punny, thank you.
In the hotwife community, where men allow their wives to have sex with other men on a regular basis, some of the women wear an anklet with a small symbol that indicates they are, indeed, a hotwife and are looking to hook up for some incredible sex. In addition, if they are also sporting a ♠ symbol on their anklet or have a small tattoo of a ♠, it means they either prefer or are open to having wild nights of passionate intercourse with well-hung black males.
The ♠ symbol is also used outside of the hotwife community, usually as a tattoo, to show a woman either exclusively dates dark-skinned men or is open to sleeping with them, assuming they have the correct sized equipment for the woman's requirements. For black men who are looking to hook up with a caucasian woman, this is one method they can use to see if there is any interest.
While this node just discusses the symbol, the whole hotwifing and open marriage culture is a book unto itself. One of my best selling erotica series, published under a pseudonym, is about hotwifing with a woman who has this tattoo.
Iron Noder 2017