Since perennial poppies are early season bloomers, it is possible that excessive spring rains or wet weather could rot the flower buds. I am not aware of any specific diseases that these plants are prone to and I have never experienced this phenomenon myself. A sunny, well-drained location with good air circulation can help.

White oriental poppies are not exactly uncommon - there's several cultivars on the market - so you could replace it rather easily if you wanted or needed to.

I've had Oriental poppies go black and shrivel before also. Have never tried to find out exact cause.

I had that problem last year, and found that earwigs were crawling into the unopened buds and eating the flowers before they bloomed. We used sluggo plus to control them and haven't had any problems since then. Good luck!