Disclaimer: All recognizable characters and elements from the Harry Potter series belong to J.K. Rowling. No copyright infringement is intended.
Blaise has no memories of his father. He has photographs of a man smiling next to his mother, who's holding a baby. He has photographs of that same man cuddling him and laughing. He sneers when people mention his father, his contempt for the man who was too stupid to escape his mother's trap obvious. But he keeps the photographs in a box with half a dozen magical and physical locks. He would never, ever admit it to anyone, but sometimes he thinks that his father must have been special. His mother must have loved him – after all, she didn't bear children for any of the others.
His earliest memory is of stepfather number one patting his head and offering him a puff from his cigar. His mother caught stepfather doing that and slapped him. One day he wasn't there to help Blaise into his special chair at the table. He wasn't there the next day either, and Blaise forgot he'd ever been there.
He never met stepfather number two. His mother left on vacation to France, where she met and married an elderly wizard within a week. That stepfather sent him a nice letter, which Blaise's nanny read to him. He was too preoccupied playing with the envelope to listen to her though. His mother returned a month later, dressed all in black and bearing him lots of presents.
Stepfather number three didn't like Blaise very much. He never spoke to him, but Blaise overheard him telling his mother that Blaise needed a little brother or sister. Blaise sprinkled salt into the man's tea every opportunity he got. Though Blaise's mother eyed him suspiciously, she never rebuked him, and the house elves were punished for the offense.
Blaise loved stepfather number four. He asked Blaise to call him "father" and Blaise happily complied. When Blaise's Hogwarts letter came, Father took him shopping for his school supplies. And although First Years weren't allowed their own brooms, Father bought him a broom for Christmas and made him promise to leave it home until next year. When Blaise arrived home for the summer, he discovered that Father had died and he hadn't been told because his mother didn't want him distracted from his exams. Blaise set his broom on fire.
He found out about stepfather number five from the Daily Prophet. Pansy read the society pages every day, and one morning she told him that his mother had married a very wealthy Indian wizard. Blaise continued eating his toast without replying. Draco teased him because there were rumors that the wizard's ancestry wasn't as pure as he pretended. Blaise ignored this, too, though he felt vindictive pleasure when the hypogriff mauled Draco.
Blaise is rather surprised that there hasn't been another stepfather in the last three years. He supposes that his mother has already used all the wealthy, single wizards who weren't immune to her charms. The ones left are very careful to test their drinks for love potions. And his mother has a new hobby now anyway. With all the time she spends in secret meetings with Death Eaters, she doesn't have time to hunt for a new husband. Blaise is relieved to hear Draco complain about his Aunt Bella's relationship with the Dark Lord. He'd rather not have to call Lord Voldemort "father."