Translation:The black trails are the most dangerous.
Only for skiing or maybe fencing. "Black diamond" got mostly the brand name "Black Diamond", but "run", "ski trail/piste", "ski area" x 2, "trails", "section" x 3, "Trail", "runs", "trail", . . . Attempting to introduce foreign terms mostly leads to a proliferation of translations.
(Double) Black diamond is used as a ski-slope rating in North America, Australia, and New Zealand (developed by the Walt Disney Company starting in 1965, and adopted by the N.S.A.A. in 1968 -- http://news.outdoortechnology.com/2015/02/04/ski-slope-rating-symbols-mean-really-mean/ ). Europe and Japan just use the colors, not the shapes, though Diamant Noir (maybe referring rather to carbonado) is a popular classy brand, name for restaurants, and such . . .
I've heard phrases like "going off piste" and I've never skiied! It's now used to describe anyone taking the road less travelled, or just not on the public footpath ;) But yeah, even I know that "piste" means the way down the mountain, with it being interchangeable with "run", but the colour-coding seems to be stuck more to piste than run (though I have heard "black run" before now). I've just never heard the word "trail" being used for any ski route which isn't cross-country (my big sis used to do that sort of crazy stuff lol). This is what happens when words get into general vocabulary.