Responses
I think the black mirrors form not just by their malice, it's also because of the setting. The room itself is full of negative energy from all that horror movie and negative feeling people place in that room. And also since mirrors can be portal to paranormal things, it basically turned into a haunted mirrors that reflects peoples dark side.
Just one thought though. Can normal mirrors turn into "black mirrors" just because of one's malice? I'm really curious. Because I don't know if I didn't understand or if it's not specified how "those" mirrors were formed. They're just like "that."