It's when you get a blowjob while you're on the toilet taking a SHIT.
Blumpkin is a gag started by Nova during The Ex-Communicated Series. He said it was having someone perform fellatio on a person who is on the toilet defecating.
It started in The Ex-Communicated Series when Nova said that the villagers should trade sexual favors for emeralds, he mentioned a Blumpkin. Both Sly and Seamus didn't know what it was, in the end Nova told them, Seamus was disgusted while Sly couldn't stop laughing. Later on in the Minecraft series Sly asked Seamus if he wanted one, he agreed and they did it outside, they left the poop in some dirt and warned Nova not to go near there. After Ex-Com, Sly started using it in other series such as Ghostcraft when he asked if Steven wanted one, but he refused. Many of the other Creatures used it such as Danz. The phrase Pumpkin Blumpkin was also made by Nova. It was requested by fans to get defined on Sarcasm with Seamus. They had Danz on the computer looking at pumpkins, and then Seamus said he needs the toilet, it ended with Seamus saying "do you really think we were going to do a skit on that?".