Hydrangeas have the unique property of changing their flower color depending on the levels of alkalinity or acidity present in soil. An acidic soil generates blue color while alkaline soils bring forth pink.
Most soils in our area are slightly alkaline. So is our water. If you want to turn your flowers blue try applying a cup of sulfur dust around the base of the plant a few times a year.
There are also fertilizers that include aluminum sulfate to make soil acidic. Mulching with acidic materials like pine straw and spreading coffee grounds around the base can also help. Sorry to tell you this, but you will have to keep doing this forever or your soil will eventually go back to its alkaline state. You could try growing in a container to control soil acidity but you would still have to watch the watering.
I grew up in Virginia where soils are acidic, all hydrangeas were blue, and everyone there wanted pink hydrangeas – gardeners are never satisfied!
How about considering a few alternative shrub selections with blue, or near-blue, summer flowers that don’t need acidic soil: butterfly bush (Buddleja ‘Blue Chip’), golden skyflower (Duranta erecta ‘Blue Casade’), rose of sharon (Hibiscus syriacus ‘Blue Chiffon’), blue angel’s trumpet (Iochroma australe), potato bush (Lycianthes rantonetii), cape plumbago (Plumbago auriculata ‘Imperial Blue)‘, blue anise sage (Salvia guaranitica ‘Argentina Skies’), dwarf Texas lilac (Vitex agnus-castus ‘Blue Diddley’).