Heh.. couldn't help myself:
Although I still like/feel like "both, both is nice" is my favorite choice lol.
Like once they've cleared content (doesn't matter if it's on blue mage, but could be), they leave behind a growing blue magic spell (mist?). After some time for their blue magic mist to develop they can enter these dungeons with their full potential (the mist enhancing and supporting their more dangerous magics, level 5 death in turn 5 coil unlocked :3). The time to develop the mist being code for "once the devs have decided to let you in".