We found a number of these black beetles on May 20, 2012. They were in the flowers of the same bush where we found the crab spider posted last time, although not on the exact same bunch of flowers.
They look to me like small scarab beetles (family Scarabaeidae). A good strategy for finding things on BugGuide is to search for a family name plus whatever it is eating, and then scanning the thumbnails until you find something close. So I tried “flower scarab”, and looked for black ones.
When I looked it up myself, I thought it was in the genus Euphoria. But, it turns out I was pretty seriously mistaken, and didn’t even have the right scarab subfamily. Down in the comments, Margaret Thayer and Art Evans pointed me towards the Monkey Beetles in the genus Hoplia as a better match, based on details of the wing covers and abdomen shape. And of the members of this genus that are on BugGuide, the closest match looks to be the Dark Hoplia, Hoplia trivialis.
Beetles in this genus are commonly found on flowers (eating nectar and pollen). The grubs eat plant roots, and pupate underground, emerging as adult beetles early in the spring.
 “Euphoria” is an odd genus name for a group of unassuming little beetles. They don’t seem unusually happy themselves, or likely to make others particularly euphoric upon finding them (and it is pretty unlikely that they are a euphoric drug if eaten, but one can never be too sure about such things without trying it, I suppose). BugGuide has a little blurb about the origin of the name, but they are not sure whether it has something to do with moving well, or looking like a bee. Most likely, the person describing the genus just wasn’t trying very hard to have his “pseudolatin” mean anything in particular. Or maybe he had an Aunt Euphoria that he named the genus after.