A little goes a long way. While I prefer to add garden sulphur myself (that nursery is closer) to my hydrangeas, I am amending to keep the pH in check and not to turn the blooms blue. Since some potting soils will not have trace amounts of aluminum, you may do better with aluminum sulphate. Foe example, you could add 2 tablespoons of a/s to a gallon of water (always check the label directions though). And yes, some mopheads will even grow to an indecent size in pots if you let them.

You will need to add amendments to the pot regularly, (i.e., forever) if you wish to keep getting new blooms in blue.