So someone on Reddit brought up an interesting question and no one has an answer, so we're now all wondering the same thing.
According to the rules on page 211, you do a delving roll and cross a number of zones equal to the successes before encountering a hazard. Most important for this question is that according to the rules, if you cross through multiple Blight zones, you only need to check for Blight damage once based on the zone with the highest rating you cross through.
On page 213, rules state you don't need to do any delving rolls or check for hazards on the way back up.
Cool. So what about the Blight in those zones you pass through on the way back up? Do you only take the highest Blight value across ALL the zones you cross through on the way back and only deal Blight damage once?