My guess would be thrip (tiny sucking insects which damage the bloom while its still forming) or Spider mites. I use either a Bayer 3 in 1 product or Neem oil for control.

I have a particular white/yellow variety that tends to do this early in the season, I haven't figured out why. But it seems to go away with the June gloom!

Is it an established plant? Or a newly rooted cutting?

I know this is an old post but was wondering how your plant is doing now ,because I'm having the exact same problem. My plumeria (color) looks like yours. Thanks.

My blooms are turning brown and dying before they fill open. What can I do?