Hello, etl75. I am not sure I would be too worried. If the leaves are browning from the edges inwards, the shrubs need more soil moisture. But it is a little more harder to tell for sure about the blooms. At this time of the year, they tend to be mature and will begin a progression of color changes that ends in brown.
The blooms can also turn brown early if the plant aborts them due to lack of soil moisture. Hydrangeas, if exposed to soil moisture issues, will first abort flower buds then blooms and-or flowers. A lot of sunlight and high temperatures will increase the chances of browning but 31-degrees C should not be high enough to cause that problem which makes me wonder if the blooms are spent (mature).
My Little Lime and some nearby Vanilla Strawberry Paniculatas are known to get some browning on the sepals due to my daily 100-degree F (38-degree Celcius) temperatures at the end of August. Some paniculata varieties like Limelight and Little Lime, for example, are notorius for dropping leaves and "showing their legs" by the end of the growing season.
Arborescens can also loose leaves as some varieties do poorly in hot locations (especially the ones whose leaves are grayish or silver underneath). Annabelle, its cousins Incredibelle and Invincibelle Spirit I and II & the species are more heat tolerant but may still drop leaves if exposed to a "dryness". And yes, all Smooth Hydrangeas will begin changing the color of the blooms at around this time of the year.
Do consider that these were planted only months ago so transplant shock is always a problem and will act in unusual ways on their first year (bloom earlier or later than normal, etc) and especially in their first summer.