Scrying is a slightly more controversial form of divination. Throughout history, mirrors have often been considered a portal to the spirit realm, a gateway to hell, even just an evil doorway. Because of these negative thought on mirrors, scrying with black mirrors became more controversial. This type of scrying is similar to water or crystal ball scrying. The process involves darkness and a small light such as a candle, and a lot of staring, for a long time. Images come and go in the reflection and similar to all other divination, your interpretation is what matters.