The Beldam and the Black

Sirius Black loved his mother dearly, and was sorted into Slytherin. He couldn't help that he fell into deep love with a magnificent muggleborn Ravenclaw. A muggleborn that was actually from a long thought dead, but actually squib, line of witches and wizards, making their future children quite powerful. This is how Sirius Black met and fell in love with Petunia Evans. Prequel!