reverse blowjob
A reverse blowjob is a non-traditional sex act that involves a character with a penis receiving oral sex in a way where their penis exits another character's mouth rather than enters it.
The first, most realistic, and most common involves a decapitated head. The character receiving the blowjob (fellatio) inserts their penis into the stump of the neck and uses the severed head as an organic Sex Toy. This is typically accompanied with blood, death, gore and, of course, necrophilia. There are some alternative positions, so to speak, such as using one's own head, using the other neck stump, and so forth.
A second option frequently involves creatures with blowholes engaging in blowhole penetration.
A third option is if the a character's body allows for easy and safe removal of their own head, such as if the character is a Dullahan or devoid of normal biological dependencies such as if the character is a robot or a living doll.
A fourth option is through the usage of Non-Euclidean sex, such as portals to create the illusion of removable body parts.
Finally, the last way to accomplish this would be if the penetrated partner's body is capable of selective permeation of the penetrating partner's penis. This can be accomplished if the character is made out of Goo or intangible like a ghost.
This does not include the penis being inserted into the anus (or pussy), and going all the way through the body to the mouth, despite similarities.
Don't try this at home, kids!