I always thought that learning blue magic should be like this:

You're level 30, as is the spell you want to learn. You ought to have a 25% chance of learning that spell. For every level you're below the spell of the level, take off 5%. That means if you're 5 or more levels below the level of the spell, you can't learn it period (assuming capped blue magic skill).

For every level above a spell it ought to add another 5% and cap out at 50%. This means that at level 90 a BLU can go learn a level 40 spell at a 50% success rate.

For each mob killed that used the move at least once, if the BLU did not learn it, the success rate should be raised by a certain %. 3~5% seems fair to me. It -should- cap out at a higher rate than 50% (perhaps 70~90%) because I do believe that, in this game, chance should always play a part in learning blue magic. But it should not be as steep as it stands now.

And I do hope that HNM spells have an increased learning rate on them. I don't want to have to kill over 50 Genbus to learn Aqua Breath (hypothetically speaking - something tells me we won't be getting that spell, even though it is a FF blue magic Classic).