I don't use wood mulch around my Azaleas or Hydrangeas. I use pine straw which adds acid to the soil.
You can fertilize once a year slow-release fertilizer, or fertilize twice a year with 10-10-10. . Stop fertilizing by August to let your hydrangea get ready to go dormant.
But what I do is feed them once a month April thru July with Miracle-Gro Azalea/Camellia/Rhododendron Plant Food. This fertilizer is 30-10-10.
I mix it in a watering can. I use this because I happen to have it on hand for the Azaleas and my Orchids. Works well for mine.