Met in a muggle elementary school, Blaise started showing signs of magic in grade 2 so his mother pulled him out and signed him up for a magical elementary school(he meets Draco and Pansy there). In Hogwarts, harry meets Blaise again and they both remember each other, but harry is jumpier than he remembers, he gets his mom to look into it, and finds out he's abused. Long story short, Harry's now his little brother.