Actually it's not just blazed faced ones. Husky marked, high whites, etc. can also carry it. Here's a great link for reading about megacolon.

http://ratguide.com/health/digestive/megacolon.php

In our particular case it was early onset megacolon. The pup was just starting to eat food (as well as nurse of course) and it was visually obvious that is was a megacolon offspring.

Having said this, one of my best producers is a Husky marked female and she's never produced a megacolon offspring. I have though been careful to pair her with only low white males with no blaze faces and never to another Husky just to be careful of that risk factor. It's handy have multiple male breeder rats.