What is so special about using a black mirror over any other mirror?
well a black mirror is meant to be your magick mirror used only for skrying. it's a tool and therefore should be used for magickal purposes only. [also the back is blacked out hence the name]
Can I just use my regular on?
you can use any mirror of your choosing, but again it's best to use it only for its intended magickal purpose instead of reflecting mirror by day, magickal skrying mirror by night.
Can the mirror open door ways to another dimension for contact?
depends on what you mean, a physical being won't come through the mirror, nor will you be able to go through it either. i have heard of a cleansing ritual with a blacked out mirror that takes negative energy to a different place, but i don't think it's another dimension.
I've heard you can contact people through them with a ritual but why does it have to be a black mirror?
black mirrors basically remove the reflection [kind of like a turned off tv, you can still see things but not as perfectly as a bathroom mirror] you can use many things to skry like a crystal ball or bowl of water, but a black mirror is considered the easiest to use because you don't have to struggle to look beyond your own image as much as other methods.
Are there special qualities in it that are not found in any other?
in any other mirror? again, a black mirror would become your skrying tool. magickal tools are blessed and charged with energy so they are ready to be used for their designed purpose. it is this energy and blessing that makes them magickal [an athame is just a knife until you choose to use it for directing energy in circle] black mirrors should be used for skrying and covered when not in use because that's their designed purpose, skrying, not putting on make-up in the morning.