I heard that it's not so much the steeps are that much steeper or the blacks harder, it's that they're mainly a lot longer out west.
Flame on, Westies!
No, they are steeper although they generally tend to be wider and more open with less trees/moguls (not always true though). Here is an example photo of a double black diamond. This is the "drop in" for the Wall at Kirkwood.
Squaw's KT-22 only rates a single black diamond and becomes blue half way down.
That's not to say there aren't challenging slopes elsewhere... just there are more out West as the mountain are bigger. I grew up in New England any moved out West after college.