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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
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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
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