You don't KNOW the scaling, you can't say its terrible. You're also leaving off Off-Guard which is another 50% increase AND you're forgetting that Blue Mage spells cast around 20% faster than black mage spells. You're also not factoring in the power of Primal Spells...though for some reason Bristle doesn't work on those so admittedly they aren't hitting for as much as they SHOULD be. But that's another problem in and of itself, BLU really needs those spells to even be close to competitive but farming anything but eruption requires a group.
Blue mage single target damage is low, though. Not as low as you are making it sound, but still low. BLU either requires targets to be weak to status ailments that every boss is immune to, or requires a heavy time and party investment in order to be...sub-par on anything other than trash mobs.