When they first come out of dormancy they need nitrogen. (careful with the form of nitrogen for blueberries). I water as needed (soil probe) but generally as soon as I see bud swell I will check to soil because they will start using water too. If using synthetic nitrogen I'd dissolve into the water before applying per directions. If using organic nitrogen, I'd apply to the well/rootzone just before watering.

FN's advice seems good to me. By type of N, he means urea or ammonium and not nitrate for blueberries if you are going quick release.

Fertilize according to need, some soils don't need much help for some species (pears are more susceptible to fireblight when growing very vigorously).

If trees or bushes are mature and you are feeding for fruit and not leaves and wood, quick release N is best, about a week before first growth (although the window extends a couple weeks). Urea doesn't need to be dissolved but does need to be watered in. I apply right before rain when possible.

Commercial growers often use urea made for foliar application and spray the trees to get N directly to spur leaves at first growth.

I think the best source of quick release organic N is your own urine, which is also very high in K. I save mine for the purpose but in our sterile modern world this is often not acceptable. My wife is certainly not crazy about the idea.