Bellatrix Black is Sorted into Slytherin house, like the rest of her family has always been.
Bellatrix was born in 1951 (BFT) which means she would have started Hogwarts in either 1962 or possibly 1963 depending on her birth month.
Bellatrix Black starts Hogwarts
Date September 1st, 1962
Certainty Inferred from canon
Location in Canon BFT: The Black Family Tree
Type of Event Wizarding world