Alphard Black was a Pureblood Slytherin during his time at Hogwarts and a canon character

Daniel Day-Lewis as Alphard Black

in the Harry Potter books. He returned to Hogwarts in 1975 to teach Defense Against the Dark Arts to the students, one of which was his nephew, Sirius Black. While Professor Black was still a Slytherin like almost all of the Blacks, he did not agree with most of their beliefs, and was the closest thing to real family Sirius ever had.  He died peacefully of terminal illness on May 1st, 1976, leaving almost all his fortune behind to Sirius