I've been having a lot of problems with white marks on the dark velvety falls this year, too. It is really irritating. I'm not sure it is rain, because we haven't gotten that much rain, but I guess it might be dew. It's not just speckles, either, but streaks as well, and it is not all the dark irises, just some of them.
I would sure like to figure out how to prevent it, but