Here is a simple method for making a black mirror at home.

Buy a picture frame, complete with the glass.

Pick up some black paint at the craft/hardware shop, best use gouache paint, acrylic or oil based. Make sure you have a paintbrush!

Disassemble the frame and lay the glass on a piece of newspaper.

Paint the backside of the glass with the black paint and let dry.  I recommend 2-3 coats.

After it is all dry, if there is any need clean the outer part of the glass with window cleaner.

Place the painted glass into the picture frame the unpainted glass side facing outwards.

~from Maat's Book of Shadows