Ok, I just finished my scotch, and it has straightened out my thinking.
It's really pretty simple.
The runs are rated as per RELATIVE difficulty compared to the rest of the hill. If you skied their blacks and they are not challenging, you can ski their double diamonds, without too much danger, at the worst you will find them not fun. If you can ski their double blacks without worry, then the tripple blacks should not cause undue difficulties.
All you really have to do is see how they scale their ratings by skiing their runs. If you ski black diamonds and they scare you, stay off the double black diamonds.
What you say here makes sense, but I have seen quite a few situations where this just isn't true. I've seen blues that were more difficult than the blacks at the same mountain. It also doesn't address the variation within a category. For example the easiest blue and the most difficult blue on a mountain both look the same based on their rating. For a skier moving up from the greens, it would be nice to have a little bit more information available to be able to tell them apart.
What you say is true, and there are a few exceptions (I can think of one green that is too much for people moving up from the bunny hill too), but for the most part, any of the next level trail for someone moving up will not be a disaster. I mean it's not like you forgot to pay attention to the trail signs while leading your intermediate daughter down a blue that turned into an icy black.