It is believed that Sirius Black was one of the people whose Boggart assumed the form of Lord Voldemort. However, it is also speculated from the understanding of Sirius's character that his Boggart might also have been a different version of himself. More specifically, a version of himself that his family wanted him to be. Sirius feared the possibility of being a Pure-blood fanatic like the rest of the Black family and carrying around the Dark Mark on his hand.