Technically this depends on a resort - in Aspen or Squaw Valley a double black diamond is in the "insane" level category, much steeper/challenging than even the black FIS slope in Sölden. I remember the lift sign in Squaw Valley's Headwall/Palisades - "Experts only. No easy way down.", and it was like that. Maybe upper part of Hahnenkamm could be comparable ;-) Usually double black diamond corresponds to European free ride zones near steep ridges.

Of course. Trail ratings in North America are relative to each resort. I've gone down blues in the west that are far more challenging than the double blacks at a few areas in New England.