As opposed to the blumpkin, in which a dude gets his cock sucked while taking a dump, a bluffkin is:
a) the act in which a dude believes that his lady is going to suck his cock whilst he is taking a dump and instead, she leaves him alone such that he only takes a dump el solo;
b) the act in which a dude convinces his lady to suck his cock while he takes a dump, but doesn't actually poop.
1) I was totally gonna give my boyfriend a blumpkin, but he never pooped and thus, bluffkinned me.
2) Dude, my girlfriend gave me a bluffkin last night. I was so ready to shit and cum at the same time its not even funny. But she denied me the blumpkin. Bitch.