If I were trying this, I'd call my local glass company that sells mirrors, and ask what solution w/take off the silver on the back, then I'd probably sand it a bit, and sorta dry brush/wipe it on lightly. I'd probably use a little piece of scrap mirror for a test piece. I use lots of mirror in my mosaics, and usually get freebie scraps from my glass company.

http://www.finishing.com/3800-3999/3915.shtml
Read down on this page, gives alot of info on this.

I tried this by sanding off the coating and spray painting the back black. The main problem I had was that even tho I thought I sanded enough I didnt and some scratchy areas showed. Buy an old mirror at a yard sale or something and fool with it...see if you can come up with something you like.