Genevieve and Xander Blackbourne were a perfectly normal muggle couple. Straight as rods, businesspeople, Christian, relatively well-off, proud parents of a four-year old son.
Genevieve gave birth to a second child a few weeks after their eldest son (named Xander after his father) turned four. They planned to call him Calvin. And then he was female. They let Xander Jr name the newborn, and he decided that Nutella would be a wonderful name for his little baby sister. The recipient of this name disagrees. For some reason, Genevieve and Xander decided that Nutella was an acceptable name for their daughter, completely disregarding the fact that she'd most likely be teased endlessly at school.
Ella attended school from the age of five, much like anyone else, and she was your typical bubbly child - she made plenty of friends, played bulldog in the playground until it got banned, then continued playing it under a different name, traded Moshi Monsters cards, treated the benches at the back of the assembly hall like the iron throne, covered her hands in PVA glue then peeled it off when it had dried, etcetera etcetera.
After she left primary school (and had the names of aLL the students and teachers and dinner ladies and PTA members on her polo shirt), Ella received her Hogwarts letter and began school, being sorted into Gryffindor. Her first year was fairly uneventful and she excelled at most subjects, though especially the more theoretical ones. She also endeavoured to keep up with basic English and Maths and Science and languages (okay, mainly Spanish, but still), since she wasn't entirely ready to give up all 'conventional' schooling.
The summer after second year, Genevieve Blackbourne accompanied Ella alone to Diagon Alley, and they had a great time. That was until they took a wrong turn, ended up in Knockturn Alley, and got caught up in a mugging-kidnapping incident. And thus Genevieve and Nutella Blackbourne disappeared.
The idea the kidnappers - a group of wealthy young dark wizards - had was to attempt to create a 'human time turner' out of Genevieve, whom they assumed was a witch. Ella was there only for insurance. That was until her mother died during the experiments and the kidnappers finally realised she was, in fact, a muggle. And then they turned their attentions to Ella, who they knew for a fact was a witch.
Their experiments on Ella didn't go exactly to plan, but she was close enough to what they'd wanted to be useful. Naturally, they taught her things - how to go unseen, how to lie through her teeth, how to steal and pick locks and other things. They didn't get to make her use these newfound skills for long, however, because she picked the locks, stole her wand back and disappeared just before her fourteenth birthday. Naturally, she found her way to the ministry and brought the force of the aurors down on the kidnappers as "human traffickers". Moral of the story: don't teach teenage girls how to pick locks and lie convincingly.
Ella returned to school for her fourth year, and kept up with the work only because she'd had a sort-of tutor for the past two years. As was to be expected, she'd found a sudden talent in History of Magic. In her fifth year, her father remarried to a witch named Eleanor who had her own children, Freya and David, and life at home took on a semblance of normality again. Eleanor she strongly disliked, because she was so different from her real mother, being strict and uptight, with practically no personality. Not motherly in the slightest. She pitied Freya and David that.
Upon graduation, Ella became History of Magic professor at Durmstrang until she was 24, when she was offered a position at Hogwarts. Naturally, she took the opportunity immediately.