At one point I made a t-shirt with the image in my default userpic (used in this comment) on it, and I wanted to come up with a slogan in Latin to go with. I thought it looked like an angry kitten, but there was no classical Latin word for kitten. Modern folks seeking to fill that gap have used the construction "felicula" for kitten. I liked the sound of that, so I emblazoned the shirt with the words "felicula inritata," enraged kitten. About the same time I was working on the shirt, I decided I could use a new username. The whole phrase was a bit too long, but since I wear tons of black, blackfelicula fit.
I am a cat lover in general, so the username stuck. I've been using it in a few different places since before I started this journal... 2001 I think?