@Mythopoeia I thought this was the spell for a black mirror and i think it could be modified.
I got this from the Witches Spell A Day 2017.
The author is Susan Pesznecker.
A hexenspiegel (Hex - un - spee -gull) is a small mirror used as a protective charm to reflect away negative magicks, the evil eye, or other bad omens, or intentions. As well as return the negative energy to the sender.
Its basis is in German folk magick and its translation is witches mirror.
Hexenspeigels maybe fastened to walls, or hung from cords, or worn as jewerly or sitting in a special spot.
To make your own use a plain mirror or use crafts to add small stones ,crystals, beads or other items to the mirrors edge.
Charge the mirror by sprinkling it with salt, passing it through the heat and smoke of a candle and dabbling it with pure water.
Place the Hexenspiegel where it will carry out your intention, saying :
Mirror, mirror on the wall, answer my protective call. Turn away the magick black , with all your power, please send it back.
Or you could make your own words.
I think the charging part you could also do for a black mirror. I see no reason why it wouldnt work. Running it through the heat and smoke of a candle and then dabbing it with pure water. You would just want to change the intention part a little to fit the uses of a black mirror.