So, this was a surprise. I'm not sure if it's actually an Old Black Magic iris, but that's the closest I could find when I searched images on line. The petals are more black than burgundy when not photographed in full sun. This iris is one of the dozens that I dug up right after I bought the house a couple of years ago, as they were right smack in the way of foundation work being done. Below: The mystery bunch of irises (and daylilies) I inherited with the house, in their original spot.
I have yet to figure out the secret of getting irises to bloom: am I using too much bone meal? too little? are they planted too deep? Exhibit A: these were planted last spring, got a bit of bone meal in the fall, have beautiful strappy foliage, and no sign of a bud anywhere.
But each rare time an iris does bloom, I think: "Oh, so that's the color I planted there." This time, the reveal was delightful, especially next to the pale orange flowers of the Indian mallow.