I have been practicing with my own black mirror and can tell you in over a month, I am only recently starting to see things in it.
First, the JoS witchcraft page had a link for black mirror empowerment, but it also described the learning curve, you start by learning to create ectoplasm for things to manifest in. That takes time and then it will at first only be cloudy, that will in some time start to become basic shapes, lines and curves coming through the black mirror. Then you need to keep trying and will be rewarded, I recently had my first vision in my black mirror, I saw the clouds, those became swirls. And that turned into a vision of the local and largest church in my town going up in flames and a panic filled market square beneath the church and that vision rolled over the city center where that church is and faded. Ever since that, I think is now a week or two ago, I haven't been able to get anything but the clouds in my black mirror. As that JoS page tells, at first the visions will come at random so don't expect them every time as you progress, and with practice at producing the visions you will learn to have them on command with just looking into the black mirror briefly. I have to add, all work with a black mirror is done in trance so also keep on working on that, my only vision was also my deepest trance so far with my eyes open. You have to train a couple of things for a long period of time to get anywhere with a black mirror.
Sorry if I sound a bit negative in parts of this, just trying to clear up a few things for you.
HAIL SATAN FOREVER AND ALL THE GODS OF HELL