To be exact, Blossom multiplies growths by a factor of 10/9 (approx. 1.11...) in FE9.
Even if it's 20% (or more >_>) I wouldn't consider it significant, because it makes low growths still low and high growths higher. I suppose it's good for getting to a point where you can BEXP their lower stats better, but I honestly prefer the 2x experience from Paragon.
You're forgetting that while low growths are still low, they're higher than they would be otherwise. So, not only does the unit reach the point at which you can start BExp-ing their lower stats earlier, but their lower stats would already be possibly one or two points higher by that point, meaning they'll cap those stats, too, with less effort and less BExp.
Now on the hypothetical case that it raises growths at a fixed 20 percentage points, I'd personally love that skill; 30% is a hell of a lot better than 10%.