The blue sheen is only seen in proper lighting. What told me these were tree swallows is the strong distinction between dark head/back and light belly. Looking at the second photo, you see this clear distinction. The only other swallow in your area that would have a similar look is the violet-green swallow, but their eye would be in the light area, whereas the tree swallow has it's eye in the dark area. Glad you found the link helpful, Cornell has a great website for IDing birds! Glad to help :)
On the 3-4 birds I saw, I didn't see the blue sheen on the back and the tummy was not that pure white as on the pictures I found when researching tree swallows. However it could be a female or a juvenile. Also according to the link the Life History fits very closely to what I saw (that was a very useful link, thanks!), so I think it is a tree swallow.
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