Moved from American Bluets.

To me it looks most like River Bluet, except for the pattern on S2. They are somewhat blurred, but the cerci look pretty long to me. The black on S2 might be the result of some variation that is not mentioned in the books.

now to the ID. We would propose this as a damselfly in the genus Enallagma. Luckily there aren't too many with the color pattern consisting of a mostly blue abdominal segment 3 changing gradually on each successive segment to an almost entirely black segment 7. And most of those are eastern species. The wide black shoulder stripe on your image eliminates E. clausum. Which leaves us with E. anna and E. carunculatum. E. anna should have noticeably huge claspers on the male and should be almost equally blue and black. Abdominal segment 5 would be about half and half black and blue. So we will suggest this is E. carunculatum with its smaller claspers and with more black than blue on the abdomen - segment 4 is the half black half blue one.
We must add one caution - the black color pattern on segment 2 with black color coming down and then forward is typical for the genus Coenagrion, but we don't know of one with so much blue on segment 6. C resolutum should have none there and a green thorax, while C. interrogatum should have some blue on the back end of segment 7. That's why we're choosing Enallagma, but we must admit segment 2 bothers us!
This is a fascinating beast! It will be interesting to see what Cliff has to say. Thanks for posting such a remarkable specimen.

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Sorry, wrong location. Yellowstone Wyoming. I'm so used to typing in Texas I forgot.