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.
"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."