Transplant shock sometimes causes this issue as summer approaches. The heat and loss of water via the large leaves makes the plant take proactive action. That action can be to abort the flowers and brown them out. I things continue hot, they could wilt the leaves to reduce the amount of leaf surface area that is getting hit by the sun. Or the plant can make the leaves brown out from the edges inwards and then they fall and get replaced once temps moderate or next Spring.

The rock bed absorbs and retains heat so once it is night, the plant remains a tad stressed even longer until the heat dissipates and there is no more problem. I would recommend organic mulch instead so heat absorption by the rocks and sun reflection from the rocks to the plant are no longer a problem.

Since summer is around the corner, consider watering about 1.5 gallons per watering if a finger inserted into the soil to a depth of 4" feels dry or almost dry. Water the soil -not the leaves- always early in the mornings.

Your shrub is newly planted as you said. Its root system was pruned to fit on the pot. That makes the shrub slightly more sensitive to summer issues but they usually get better in 1-3 years. If you notice wilting, there are a few things you can do and not do:

If you see a very ugly wilting episode, give the plant about 1 gallon of water immediately. But otherwise, first check the soil using the finger method. If the soil is moist or wet, the plant will recover on its own at night or before morning. If the soil is dry then of course give it some water.

Make sure you have 3-4" of mulch to make the water stay in the soil and prevent it from evaporating. Add the mulch up to the drip line.

Note: there is also another possibility that you could check and rule out. A mold called gray mold can also develop on the blooms and make them brown out. When that happens the sepals feel paper-ry and may look "wet". Here is more info on the symptoms: "symptoms are usually limited to the flower buds and especially the petals.The small water-soaked spots seen on the petals quickly expand into reddish brown irregular blotches. Brightly colored petals quickly fade to a brown, withered mass that is often covered with the fuzzy gray growth of the causal fungus Botrytis cinerea. Masses of gray spores can easily be seen with a hand lens." You can read details in here: http://www.aces.edu/pubs/docs/A/ANR-1212/ANR-1212.pdf