What is my pizza cutter's handle made from, and how do I stop it from staining everything it touches?
Mystery solved!  At Catija's prompting, I managed to dig up the cutter on the website of the shop I bought it from, the Chef's Hat in Melbourne, Australia:



CUTTER PIZZA W/ALUM HANDLE 95MM S/ST ($6.60)

So turns out the handle is aluminum (aluminium), which can be discolored black if the alloy is not dishwasher-safe, which this clearly wasn't.  Time to ask a new question about what, if anything, I can do to fix it!