Blair Aberlour Hidden History
The history that Blair doesn't know WARNING SPOILERS. Their mother (El's and Blair's)is in fact a little out of touch with the way things really are due to what Blake has done. Where she got mixed up with Blake Archer while still with El's dad, in the end she did in fact run off with Blake. She left with Blake because he convinced her muggles thought she killed her child, the infant Elodie, and would come after her. That left her with a dead child and abusive husband which made it easy for Blake to convince her to run away with him in the heat of the moment. They staged her suicide, leaving her scarf on the edge of a cliff, and left.
So she had to run, and he would help keep her safe if she went with him. She believed Blake because her first husband was abusive. Not wasn't solely abused by her muggle husband because of her metamorph magic, that was a huge factor but he's also simply an abusive man who didn't really need the excuse to be a bastard.
So she runs off with Blake, they start a life somewhere and he mostly does it for his own gain, hoping for a son to let the name live on and having metamorphmagus in the family was invaluable. And for years they tried to have a kid, Blake made sure to keep his wife happy so she didn't see who or what he was really like.
Aurelia was pushed in her grief to set all blame on muggles, the supposed incident proving how much of a pest they are to the wizarding world and how superior the wizarding world is in its destined purity. By the time she had Blair, who was supposed to be a boy to carry on the Archer name, she was neck deep in company of dark wizards and death eaters, herself having joined their ranks. The new child was nothing to her since it wasn't male, a liability that needed to be disposed of, so she left it. Aurelia their mother sees Blake as her savior and would do pretty much anything to please him.
When Aurelia dies, the group realizes how much of an asset a metamorphmagus is and decide to check on her daughter, having kept an eye on her through the years. They not only find her but that her older sister Elodie is in the castle too, and another metamorphmagus at that, so they try to sway Blair to join them. Probably in hopes that her sister will follow. Blair also poses a potential as producer of a male Archer heir, something her mother failed to deliver. Adding to the strength of their pureblood line. Can he convince her to side with him? Will she fall to her dark bloodline?
During this time her father unknown to her was looking for the heir that he could use and see to the continuation of his pureblood line. Working His way into the Courtroom so that he could truly have all accesses to what he wanted and needed. And so as far as he could see no one could stop his plans. Blake was at gaining access to the top, and the power he wanted. No was time to find the girl and finish what he started with her mother.