Similary to a golden shower, a dark treat occurs when one deficates on the face of his or her partner- typically following intercourse. Popular in homo and heterosexuals alike.
Of course a dark treat can occur without sexual intercourse or any other related or non-related sexual play.
In a homosexual sense: After Nick bunged Bill, Bill was so loose he gave Nick a dark treat.
In a heterosexual sense: Jackie was feeling kinky after taking her first cock in the ass she decided to give Josh a dark treat.
In a non-sexual sense: Kelly is so disgusting. She was so drunk at that party she let Ben give her a dark treat in front of everyone!