Welcome to NGA, tangyue. When you have hydrangeas, blooms usually go through a series of color changes that –for example- start with blue and end with brown. When you see that blooms are going to brown directly (prematurely browning), consider heat stress and transplanting, especially in newly purchased hydrangeas:
• Foliage and flowers on new plants cannot always stand direct sunlight as well as they can in established hydrangeas. That is because the plants were developed in optimum conditions that include very bright but indirect light. After these photo sensitive foliage dies off –now or at the end of the Fall season- new foliage should be less sensitive. It also helps sometimes to gradually acclimate the plant to more sun by putting it first in bright shade, then moving it to 1 hour of sun, then a little more sun and so forth but, make sure they get shade starting at 10-11am. Or provide more shade somehow. Just aim for shade in the afternoons and evenings.
• Insufficient water or lack of water can also cause browning of blooms. If the problem persists, the leaves can also brown from the edges inwards. Water when the soil feels dry or almost dry if you insert a finger into the soil to a depth of 3-4"; skip watering otherwise to minimize the chances of over-watering and developing root rot or other leaf fungal infections. When watering, never water the leaves; try to only water the soil as best as you can. Once you have watered, use the finger method in several spots at a depth of 5-7" to see if you watered enough. If a spot feels dry then you missed that spot or did not have enough water.
• Make sure that the water drainage holes are not clogged to prevent root rot problems. The symptoms of root rot are almost identical to those of not enough water as the roots decay and cannot absorb enough moisture.
• When temperatures typically run high or stay high (say, above 85F), you may notice that the leaves tend to wilt and that you need to water at "Summer watering levels instead". While it may look awful, sometimes wilting is harmless. It is a defense mechanism in which the leaves wilt/droop and reduce the amount of leaf surface being hit by direct sun. It happens when they lose moisture through the leaves much faster than the roots can absorb more water. As a result of environmental conditions, the leaves then wilt. When I see a hydrangea wilting, I use the finger method at a depth of 3-4" to check the soil moisture; if the soil is dry or almost dry, I water immediately. If it is moist, you could just leave the plant wilted and it should recover overnight on its own once the evening hours arrive. If you see the planted and decide not to water because it felt moist, check if has not recovered by the morning. If it has not then water it. Optionally, you could water it as soon as you see it wilted but try to control yourself as too much watering for long periods of time may cause root rot. If the wilting episode looks the worst ever, I immediately water. Start watering using 1 gallon of water and tweak that if the finger method at 7-8" of depth says that the amount of water was not enough.
• Wind - this type of hydrangea is called a big leaf hydrangea for a reason and it is those leaves that help lose moisture so when you have windy conditions, the wind can help the leaves lose moisture. I sometimes water the night before a windy day (if the NWS announces a wind advisory). Winds, heat and-or sunlight can make the large leaves lose moisture faster than the roots can absorb the same amount of water.
• Mulch helps retain soil humidity and protect the roots from temperature extremes (too hot and too cold). When planted in the ground, I give them 2-4" of organic mulch (no rocks).
• In Year 1, some people increase the amount of shade by moving the plants to a more shady location, adding a wind block of some type or by using another object to project shade on the plant. Think: outside chairs, umbrellas, cardboard contraptions, etc. If it is getting sun after 10-11am hours, I would look for an alternate location before summer temps arrive. You may get a taste of this when temps cross the 85F barrier around June for Washington, DC.
• Hydrangeas have shallow, tiny, fibrous roots in the top 3-4" of the soil so transplanting can disturb the roots if one is not careful. That area of the soil also dries out fast. The plants might respond to all that by zapping blooms and-or leaves.
Does that help you, tangyue? Luis