The black stuff is the lovely combo of compressor oils that made it out of the snowguns and a winter's worth accumulation of dirt blown onto the slopes. Hot wax with any cheap all-temp and immediately hot scrape to strip it off. Works much like leg waxing. You'll see the scraped wax is filthy. Repeat 'till it's clean.
I was told by my local shop owner that the east coast resorts run diesel gas through their snow blower lines to keep them from freezing, and that causes black stains on the bottom of the board when the snow is melting.