In Mexican culture, a slang term to to refer to the act of placing your mouth into contact with your partner's asshole. It can range from a simple kiss to more intricate making out such a licks, bites, sucks, tongue etc. i.e. a rimjob.
Literally translates to a "black kiss."
Girl 1: I gave Jason a beso negro last night. It was hot.
Girl 2: Oh yeah! I give my girlfriend besos negros all the time.
It is advisable to use a latex barrier when performing a beso negro on someone for the potential transmission of Hepatitis A, unwanted bacteria, and other STIs.