I have never noticed that effect at night... even after a rain. But who knows if something has been sprayed on your Apple Blossom hydrangea. You can try (in the dark) to cover it with half a box, thus blocking sources of light. Then determine what you see.

I cannot find any source that shows a registered cultivar of hydrangea with the name 'Apple Blossom' - do you have any additional info on this plant? Or a photo?

FWIW, white flowered hydrangeas (if this in fact IS white) and for that matter, just about any other white flowered plant, are often extremely reflective at night, some even more so that others. Often to the point where the flowers will seem to 'glow'. This is the concept behind "moon gardens" :-))

If it's a very pale colored flower, it's probably reflecting the light :)

I do not think there is an 'Apple Blossom Hydrangea' per se (watch someone find it now!). I have been at florists places and on several years, they often called my attention to specific cut hydrangea flowers & arrangements by calling the shrubs/flowers like that.

We have determined it's reflective light. Thanks all for the help.