I just made a test with a no name metallic gel roller pen. It blocks the light completely. It is a 2 USD pen that was bought for my 3 year old daughter at the postoffice. I think Your should be able to that kind of pens everywhere. It takes some time for it to dry. Another possibility is to ask at an artsupply, they might have a pen that is better suited.
I talked to a local artist supply and they sell a "lead" grease pen, that can be used on glass and other smooth surfaces. They said it will block light. You can remove with any de greasing agent (whatever that is). I have ordered one and will test if it works.
You could also do this on the computer, if you wanted to have your sig in back, and a nice border around it.
Just sign a black peice of paper a few times, scan in the one you like with a cheap flatbed, adjust your size in MS paint or whatever, and add a border. Then print it off in a transparency like a teacher uses for overhead projectors, (any university/community college library copy machine prints transparencies), usually about 50 cents each.
If you want a black signature on a white background, you can print the transparencies in silhouette.
I've use transparencies, for particular effects when helping my little girl make photograms.
More than a decade ago I had an embossing die made and I have used that on all Cibachrome prints. I also emboss B&W proofs and finals like this, specifying a deep white or black border (for fibre paper or Cibachrome, respectively). Why? Because I could never agree as to how I should sign my name, so an emboss settled the argument for all time — for me at least!! :rolleyes:
I never sign anything on mats, framed or unframed. Just the hidden emboss for perpetuity and let the image speak for itself.