The acidity in the soil will impact the color. When your soil is alkaline, the
flowers will tend to turn shades of pink. When the soil is acidic, the plant is able to absorb aluminum more easily and this will turn the blooms blue. Right now, most of the roots are
growing in the soil that came in the pot and it is that soil's alkalinity/acidity that is affecting the blooms. As the roots grow into your garden's soil next year then the alkalinity/acidity of your soil will begin to affect the color of the blooms.
If your soil is alkaline, try adding some aluminum sulphate per label directions. If your soil is acidic, you will not need to tweak the soil. A soil pH meter that gives readings in digits is best for measuring soil alkalinity and acidity. A soil pH of 6.2-6.5 or less will usually trigger blue blooms.
Note that, when your soil is normally alkaline, amended acidic soil tends to revert back to its original pH if you stop amending with aluminum sulphate so you need to amend basically forever. During winter, you can stop amending when the ground freezes or is covered with snow.
Normal applications are of 1 tbsp per gallon of water every other week but see what your a/s package recommends. Since the product contains sulphur, this can burn the roots if you apply too much so do not over apply.
Hydrangeas need very little fertilizers so apply 1/2 cup of cottonseed meal, compost or manure in June (for the whole year). This year, you probably did not need any as the potting medium probably contained those round nursery fertilizer pellets.
