Scrying mirrors arent just any mirror.
Most commonly they are a clack, reflective surface, to used in candle light (or moon light). You arent supposed to see anything physically there, so to speak. This will allow you to concentrate you meditative state without having any distractions.
Sure you could use a regular mirror for other things, such as sending back negative energy to the source, or reverse workings.
Black mirrors are also not supposed to be touched by sunlight(in some opinions of course) and to be kept in a dark place, used at night. But theyer also very easy to make. I made one from a picture frame. I painted the back of the frame black, (so when you put it together, its as if its a black picture) and the glass as your reflective surface. Afterwards, i charged in the moonlight. You can say a prayer, or censecrate it, charge, whatever you wish.
It took me a while to be able to use it with success. Though remember, practice and concentration is key!