Spent some time with Hoppes and a brush, turns out it was lead build up on the front face. Just occurred to me I've never fired lead bullets from my blued revolvers before. Woops.
There are still some small spots on the side, near the front edge, where the bluing is gone, but I can live with that.
And I suppose I'll just have to practice not cutting myself on this darn thing. Thank you all for the input.