Update June 25: According to Kathy Keatley Garvey and Dr. Robbin Thorp at UC Davis, this is a small metallic sweat bee in the genus Halictus, and is probably Halictus tripartitus. Thank you so much for your help!
I have a lot of these tiny bees. They are about 1/4-inch long (0.6 cm) and appear black at first glance. The one below was on a five-spot flower among honey bees and bumble bees that were also foraging in the area. I’d like to know what it is.