Happens when fish are stressed or trying to show their dominance. Many fishkeepers consider any dark marking on the yellow body of the lab undesirable with the goal being "clean" yellow at all times. Tough to achieve however!
I have a "dirty faced" yellow lab - almost black under the chin - and it is female and produces the best yellow lab fry. It's an excellent breeder, and virtually all the babies are bright yellow with strong black markings on the fins and no where else. We named it "Fred" - it was one of my first fish - and I thought it was a baby male!! Go figure!
I picked up a group of labs from a friend when he got out of cichlids and after having 20 odd fish spend 45 min in a bucket with about 4 gallons of water a few of the larger fish had "dirty faces." Went away within a few hours of being put into the tank and haven't seen it again since. That said, my other group of yellow labs which is a much nicer strain never has the dirty look, striping or any other off colouration so I think it really does depend on the strain you've got.
Avril in order to tell if they are the "best" fry you would have to raise them until the males produce fry themselves. The "dirty face" is the worst on males trying to spawn, IME. It doesn't show up at all on my females and juvies.