Seems to me that I remember one US resort that uses the green,blue,red,black,yellow system. Can't remember which one, I thought it was Taos, but no.
But actually I think double-blue is a good idea. The 3 type system really isn't granular enogh. The double-blus prob. are there to indicate trails that are steeper than most blues, but don't really have the difficulty of most blacks. There are a lot of trails around that are kind of in between. Similarily, that's why I like when they mark tails as Yellow or extreme. I mean, there are double-blacks, and then there are double-blacks (and then there are the Ski Liberty double-blacks [img]graemlins/evilgrin.gif[/img]
), you know? I like to know the difference, since I am not even comfortable with the first kind!