It's not bgwiki with info on that, it's failwiki.
And seeing as Suiram posted after it without saying 'THIS IS WRONG', let's analyze it really quick.
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certain spells such as breath-type spells and 1000 Needles, there is no target-dependent term, so the entire base damage receives the Convergence bonus. However, for spells with a component of damage dependent on the target's INT, MND, or CHR (such as Firespit, Mind Blast, or Eyes on Me, respectively), the bonus is applied only to that term which does not depend on the target. As an example, consider a Lv75 BLU with 84 INT and 74 MND casting Firespit on a target with 5 INT.
Target-variant term: floor(1.5(84-5)) = 118
Target-invariant term: 3*(75+2+floor(floor(84*0.2+74*0.2)*0.83)) = 3*(77+floor(31*0.83)) = 3*(77+25) = 306
Total Damage: 118 + 306 = 424
So, with that being said, the base damage gets multiplied by whatever the bonus is from Convergence, since Breath attacks are not affected at all by the stats of a mob. That's what I'm reading, at least, and I don't exactly have the ability to go check for you anymore.
And for your other info:
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* Initial upgrade adds both an increase of 5 Magical Accuracy as well as a 5% Magic Attack increase to the base damage of a elemental based Blue Magic spell.
* Convergence will also increase the power of blue magic spells that were originally single target.
* Max upgrade 3/3 Convergence adds 15 Magical Accuracy as well as 15% Magic Attack
* Convergence can't increase Magic Accuracy or Magic Attack of any physical spell.
Does this make it worth it? Meh, I'd rather have the AoE status thing at max, and I'd probably toss points at the spell point thingy too.