This is a very large bee that I get every summer (by the dozens) on the Obedient Plant (which my grandmother calls Disobedient Plant since it spreads so wonderfully). These bees are much larger than any other I find. They are very dark black and roughly an inch or so in length. They are fuzzy on their upper body.
They frequent my flower garden every summer and are only attracted to the Obedient Plant. I do not notice them on any other plants in my garden. The location is near woods, a creek and a very large river.
They frequent the yard in late July and August when the Obedient plant blooms.