You'll undoubtedly get differing opinions on this but most professionals recommend fertilizing trees and shrubs ONLY when the plant indicates a nutrient deficiency or testing shows the soil to be lacking specific nutrients. In any kind of semi-decent soils, plants are able to obtain the nutrients they need for healthy growth from the soil without constant human intervention and chemical fertillizers. Mulching or topdressing with compost is a great way of ensuring your soils will maintain adequate nutrient and organic matter levels and encourage healthy populations of beneficial soil microorganisms. It will also help moisture lovers like the hydrangeas from stressing out in hot, dry climates like yours.

Amending the soil is a different issue. Many folks will amend the soil to achieve specific coloration in their bigleaf hydrangeas, making it more acidic for bluer flowers and less acid for pinks.

This is also my first year with hydrangeas. Mine are in pots and I put a slow release acid fertilizer 1/2 strength in about a month ago because the leaves started to look a little pale. The one mistake I made was to top water them, while hydrangeas love moist soil they can easily develop a fungus if you wet the leaves when you water.