All the other definitions have it wrong. You are all thinking of the Inverted Blumpkin
in which the female poops and sucks dick.
The Actual meaning of a Reverse Blumpkin
is when the female sits on the toilet and poops while the male is on his hands and knees eating out her pussy.
Girlfriend: Hey I'm taking a shit, come eat me out.
Boyfriend: You want me to give you a Reverse Blumpkin?