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1.
A Harry Potter fanfiction term: fanfiction describing incestuous relationships between members of the Black family.
Popular pairings include Sirius/his younger brother Regulus magically brought back from the dead and Bellatrix/Narcissa or any combination of these four characters.
It's a moot point whether extended family members count as Blacks for Blackcest. The Malfoy/Tonks families often are, but the Weasleys usually aren't.
"Hey, did you get the link to the Blackcest fanfic I sent you?"
"I read the first line then left it...it was disgusting."
"What, don't you like Blackcest?"
"Funnily enough, I don't enjoy reading stories where popular children's characters have sex with their relatives."
by Alfredooo April 23, 2006
 
2.
Harry Potter fanfiction term where the members of the Black family are in love, have sex, have feelings towards one another.
Some popular examples of blackcest are Bellatrix/Narcissa, Bellatrix/Sirius, Sirius/Regulas,
by She who must not be named February 09, 2008
 
3.
When hawt people
Who are related
FUCK
in a HAWT way
Oh My God!!! They're commiting blackcest!!

Let's watch.
by BlackcestLover July 06, 2006