I usually don't fertilize a lot during bloom. But in pots about once a month with 21-0-0 is baseline for me. But I've also used water soluable or slow release complete fertilizer. Any of them work.

If I need rapid growth then I fertilize once a week.

fruitnut, I'd guess the 21-0-0 you use is ammonium sulfate. Could you tell me how much you mix up (tsp per gallon) for your monthly feeding...I have a 20 lb bag of it and I figured that 225ppm is 0.84 tsp of 21-0-0 per gallon water. But I'm not sure what strength to use with blueberries.

I think what I've used is about your rate, one tablespoon per 2.5-3 gallons water. So that's one teaspoon per gallon more or less.

You can find plenty of discussion by searching. One is below.

Here is a link that might be useful: 21-0-0 on blueberry

You just have to be careful of it especially on dry soil and in pots. The ground is a bit more forgiving.