The point is, blue magic spell isn't zomg super hard to learn once you have your AF hands and capped blue magic. On the other hand, job point can be very time consuming to grind. In the end, the time you spend on grinding the job point may be longer than the time it save to learn spells.
Further more, most of the BLU would probably spend their CP on a more useful category first, by the time they finally have extra CP for learning blue magic, they'd probably have all the spells unless they decided to grind CP before they learn their spells.
Even if SE release new spells in the future I doubt there gonna be enough to justify this category. Back in the past SE often release spells once a couple of months, do you think it's gonna change?