In reply to rdelf:
I've just come back from Whistler and the double blacks there were basically the same as the blacks just with a more demanding entry.. Either a drop in or a narrow couloir etc. in general the runs there were not really that hard, in particular it was tough to find any sustained high gradient. Lots of bumpy stuff though
In my experience (which only includes two North American trips) the European runs tend to be pisted including the blacks, while the North American runs don't seem to be pisted above blue. That tends to lead to the American runs being heavily mogulled where the European runs might be more of a steep flat piste with the 'blackness' being due to either sustained gradient, icyness or general poor condition or narrow sections with hazards etc.
The two exceptions I can think of to that are the tunnel in Alp d'Huez and the Wall in Avoriaz(?), which were both unpisted, but I think these are considered to be pretty top end difficulty for pisted runs in Europe.
Brackenridge in the US has an additional grade which is marked as double black 'extreme terrain' on the maps.. Those generally seem to have a cliff drop in them and are best enjoyed from a chairlift in my opinion!