Upon carrying pollen from one plum tree to another (still no bees) this afternoon I noticed the same brown patches on white that were not there yesterday. Mine had to be frost. If you had the same frost and your blooms are "on the way out", as you said, then any diseases can't start an infection in the tree.

This is also the first time I'd remember seeing flower sepals damaged that had only effected the older flowers. I'm not sure if the ovaries get damaged as you can expect the older the flower the more frost sensitivity. Fortunately the dry weather will last another couple of days (probably) giving enough time for the blooms ovaries that are potentially damaged to shed before it starts raining again.

Good insurance is to spray. I would shelter it until Sunday to keep it all dry.

edit: Oh one more thing. The plums I noticed this on were very resistant to disease. They were from last flowers of 'hollywood' plum as well. This bolsters my opinion about frost as being the external variable.

Good points in the link. Never knew before that fruits can develop into problematic situations and we can guarantee you would not want that drawing in the infectious diseases into peach tree around here. (Candy-land to bacteria in defective and still developing fruit)
Take the old ones out and rely on the backups and keep a close eye on future frosts or cover up.

The best arguments for not covering up are that there is a product "KDL" that adds to regular sprays with pesticides that knocks down the level of nitrogen in the cells that make them so freeze prone.
See testimonials
(per Bob Purvis about nitrogen/potassium balance off kilter during spring)

I checked it out and unfortunately the minimum order from a retailer for KDL in Aurora OR is two whole gallons. (edit: might have been 4)