I love that blue. Nice. I guess I love those colors that I cannot get in my alkaline soil. Maybe that is why I tinker trying to produce some purples.
If you can, inquire as to whether the plant originated from a plant nursery or not. Those that come from florists or grocery stores have a tiny plant label that does not spell out the name of the variety or give a clue as to how large it will eventually be (sometimes the plant label just says 'blue hydrangea'. Lord!)
Florist hydrangeas sometimes are not hardy to the USDA Zone where you live. When that happens, they do not bloom well. Other times, the stems dry out but return from the ground. One can improve bloomage by giving it winter protection.
But the last two years, winters have been awful and they could also be behind the only-1-bloom "disease". My usual winters are mild but, the last two included times when the temperatures went up to the 80s for days and stayed below 30 for days. These wild and "long" fluctuations interrupted the dormancy on my hydrangeas. As a result, many mopheads, lacecaps and oakleaf hydrangeas produced no blooms even though they are zone hardy.
One of my unknown mopheads had exactly one bloom too, just like yours but pink. I do not know much about that mophead though; it is an unnamed hydrangea that the prev owner had here. From what I now know from experience, it needs winter protection when the temps start going up-down a lot and stay up for several days.
So, consider using winter protection as soon as it prepares for dormancy in the Fall. Put a cage of chicken wire that is taller/wider than the shrub (helps if you know how tall it will get - assume 6x6ft if unsure). Make the enclosure wider than the shrub by as many inches as you can too. Flower buds develop near the end of stems so the more separation between the chicken wire and the tips of side & tallest stems, the better. Then fill it with mulch or lots of dried out leaves. Boatloads of leaves as this is what will protect the flower buds. Top with a cardboard and add a few bricks or rocks so the cardboard does get blown away. The cardboard should be higher than the tallest upright stems too. Again, the more separation, the more winter protection you get. Then add more leaves in mid winter due to settling. Remove it about 2 weeks after your last avge date of last frost or so (plant nurseries should be able to tell you when that is if unsure).
