It's typically more like 2.5% per skillbloom, but you're looking at this the wrong way; it's about the overall increase in DPS or in some cases, even self-healing.
Take a look at the recently bloomable
Kerria's skill:
At level 1, it's a 26% chance of triggering, at level 5 it's 36%. This is a jump from a 1:4 chance to proc to a 1:3 which is a 38% increased chance to proc and coupled with the life drain, this is a massive increase in Kerria's overall fighting capability.
And this isn't even taking skill chance trigger buffs from other girls that might be in the squad into account which just makes those numbers even higher. Skillblooms are, enhancement for enhancement, worth more than any other enhancing unit (including dressblooms) which is why they're not typically paywalled.
The only advantage dressblooms have over skillblooms is for increasing TP for units that aren't likely to see combat (i.e. Whaleship).