Please don't use a razor blade to clean your turtle's shell! If you run your finger over a finger nail, you should be able to feel something, and that's what your turtle will feel as well. Probably more, since I've read they their shells are surprisingly sensitive. I clean my smallest turts with q-tips and it's amazing how they respond to the gentle rubbing. You could cause a small nick in the shell with the blade, and if bacteria gets into it, shell problems could result.
Is the gunk mineral deposits? (If you put a bit of white vinegar on a q-tip and rub it, if it comes off that's likely what it is). I'd use a soft toothbrush to periodically clean the shell.
Oh, and happy holidays to you, to!

"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-