Keying them out later revealed that they were common whitetails (Libellula lydia -- or maybe Plathemis lydia). They really are common and widespread, so I'm embarassed that I never noticed them before. I can see the argument for calling that color 'white,' but based on the ones I saw I would still vote for 'pale blue.' The color, which is only on the male adults, is created by 'pruinescence.' Had to look that up too. Pruinosity. The quality of being pruinose. Looking hoary, having a bloom of powder on the surface. Argh, click, click click, click, okay, so 'pruina' apparently means hoarfrost, or rime.
So much for my instant theory that it comes from prunes and the bloom that one often sees on the skins of fresh prunes and other plums. Now I'm embarassed that I don't know more Latin.
Back at the pond I was trying to take pictures of the mystery dragonflies, which kept settling on the banks of the ponds, then zipping off just as I was getting them in camera range. Each one seemed to be patrolling a certain stretch of bank. In each stretch there was only one dragonfly. Once you observed one briefly, you could tell which way it would fly when you disturbed it, since it was determined to stay in its territory. (And still I never succeeded in getting close enough. Embarassing.)
Sure enough, when I looked them up, the territoriality of the males was described. But it turns out that they divide their territories not only spatially but also chronologically. They do shifts of a few hours at a time. In a study in Monterey they found that a territory could be subject to seven different males in succession.
“This is my land and here I am king! All eggs laid by beautiful ladies will be fertilized by me! No one dares rival my dominion! If any usurper tries to intrude, I will attack! Until 3:30, then I'm outta here.”