I don't know why, but these dark swallowtails seem to be especially attracted to the Leadwort. Moreover, they move more quickly on this plant than they do on the others: harder to photograph, hence the graininess here. Still they look good together!
Ceratostigma plumbaginoides is just one glaring exception to the rule decreeing that there are no true blue flowers (that we can grow?). Its easy; the primary maintenance involved keeping it in check lest it take over entire beds. It flowers for a long time and has excellent red fall color. And butterflies like it.