So, I've found out what the black cat is about, its not part of a quest, but once you get out of fort joy, it becomes all happy and dissapears, it turns into a summon spell, which from my experience is useless in combat, but is great for getting to hard to reach places, as it has 2 abilities, Feline Leap and Swap Places, both of which are pretty self explanatory, the cat can basically serve the purpose of the teleport spell without the damage, it also just happens to be cute as hell.
