NekoShema has covered the main uses of black mirros very well and I agree with her words.
As for things you has to keep in mind and and try to avoid in order to use black mirrors properly and safely, there are actually some things. But these are recommendation to use the mirror properly, I don't say that there is a life danger or anything like that when it comes ot using black / scryed mirrors. So as I said there are several tips.
Scrying mirrors are not dangerous if used correctly, you should first before using cleanse your mirror, this can be done in several ways. You may read about them in different informative sources such as : articles, answers, books, etc. Same for any recommendation of a magickal action which might be given in the future. A good place to find all of this information is Google or any other search engines.
Once cleansed you should Charge (fill) it with your energy - thismight be easily done by holding in your hands in visualising your energy passing through your hands into the mirror.
Once you have done this then cast a circle (if your not sure how to do this then there are instructions online). Whether you should cast a circle of protection, most of the time yes, but it's not completely necessary. It is advised however, because you can easily absorb what you don't want and then have a hard time "coming back" from it all. But, it isn't as though you cannot scry without a shield, you can, but that should be your own judgment, level of experience and knowledge about shielding and other ways of protection. I have been into the craft for decades, have been learning about shielding and plenty of other protective methods but yet I still cast circles since it is the safest way to avoid negative energies and beings.
Also an important thing to remember is that mirrors are kinda "dangerous" because it is incredibly easy for an energy being to pass through them. That's why I always seal the mirrors in my home with sage smoke, candle wax, and the soot end of an old match. Also it is very recommended to cover the mirror with a black cloth so the mirror will not obtain negative energy when released by you or anybody who comes to your house and occasionally crosses the black (scrying) mirror you have made. As I said mirrors easily obtain your or other objects' energy when it is released so it is extremely important to protect it by covering it with something while you're not using the mirror. I hope you understand and do it as well as you can.