Well, yes, originally Mystique and Nightcrawler were put to gether because they both have blue skin. But now can't we just put them together just because it worked in the old universe? Yes, I'm a girl, and we suck in drama like a magent sucks up stray metal pieces in a metal shop, but it made for an intersting story how Kurt was a 'good' guy and Raven was not. A son trying to redeem his mother, it's kinda sweet. Who cares that they have blue skin.
As for the genetic of the thing, we've sen some mutatnts, Polaris, Cable, Nightcrawler (blue skin) take on their parents attributes. Others we have not, Pietro, Wanda, Nightcrawler (teleport/morph). It works like regular genetics, ya know? You may have a trait that has never been seen in your family before, and you may look like everyone else, I mean, technically both kinds of cases work, and even if it isn't excat, it is only comic book science.