Types of scrying I use:
Eye scrying (self) in a light mirror
Fire scrying (with or without a mirror)
Smoke scrying (with or without a mirror)
Water scrying (in a bowl or on a mirror)
Water-wax scrying
Beginner - Intermediate - Advanced
I don't use a black mirror (the traditional type) because I was taught to just use a regular mirror. Black mirrors do have certain benefits: obsidian mirrors are innately protective; black mirrors are easier to let go of your gaze with; and black mirrors are more suitable to scrying for magic against you. However, black mirrors don't work for eye scrying or water scrying, just because you can't really see what you're doing.
The reason one might choose to use a mirror for smoke or fire scrying is because it can show you the other half of the equation. What may appear as a random shape you would dismiss by looking only with your eyes could become quite a revelation in the mirror. It's called twice-looking.