Irukandji: You solved the mystery! The picture westseattledood referenced sure looked like it (and it was an excellent guess too considering the limited state of the blossoming image I posted). But I was sure I had nothing like that in the garden.
Then your post this morning reminded me that the blossom was indeed in one of the sedum pots. Well done. I recently had a client order a large format print of that image and could not remember what it was.
Thank you for your help!