Certified GKH Gardening Expert
To be honest, fertilizing is not exactly like watering. Nutrients persist in the soil and are not always immediately taken up by the plant like the plant would with water. In other words, fertilizing times are not as hard and fast as watering time.
I would say it is best to avoid fertilizing during blooming, just because of the off chance that you add too much fertilizer and cause fertilizer burn. In this situation, the flowers will be the first thing the plant sheds if stressed so it is best to avoid the possibility.
Fertilizing in spring is always a good option as the plant will be gearing up for a growth spurt and making sure the proper nutrients are available to the plant when it needs them is a good idea.
As far as frequency of fertilizing, for plants in the ground, I would say you really don't need to fertilize any more than 2X a year. The soil will hold the nutrients until the plant needs them. It is different for container grown plants though. Early spring and early fall are good times to fertilize, just because the plants will have higher nutrient needs soon. But if you fertilize in late spring or late summer, it will not harm the plant.