With the way Soul Crystals work..

"Upon the surface of this multi-aspected crystal are carved the myriad deeds of <X Job> from eras past."

It could be implied that we learn out base abilities from the soul crystal without having to kill 20-30 different monsters. Then to unlock further abilities we go on Monster Hunter esq. quests.

They could also just re-invent blue mages from what we know into something new.