Vines' nutrient needs vary across the season, and bloom is a particularly sensitive time. Leading up to bloom, you want to make sure your micronutrients are on point. Boron and Zinc for instance are crucial if you want to ensure good set. On the other hand, too much nitrogen can reprogram your vines to think veggie and forgo fruit production all together.  Once fruit is set though, it's time to lay on the macro-nutrients. Most vines benefit from Nitrogen and Phosphorous at this time. These will help your canopy expand and ripen up all those grapes you've got coming.  If your vines need Potassium, now is a good time to apply it. Again...if they need it. I don't need to tell you how much winemakers freak out about the big K, but at this point vines are making good use of it and harvest is a long way off. You can read all about the importance of Potassium here.  Calcium and Magnesium are also important, but overdoing one or the other can seriously mess with