(Photo from here.)
Thanks to my choir friend Elisabeth, the mystery weed has been identified: it's campanula, or creeping bellflower. Some people grow it on purpose, so I guess I shouldn't be calling it a weed. On the other hand, some jurisdictions have identified it as an invasive plant. For any of you non-gardeners, that's really, really bad. It's almost impossible to get rid of because of how well it spreads by way of its rhizomes. So I think I'll call it a weed after all.
In other words, the good news is that it's pretty, and the bad news is that it will smother my garden if I don't do something about it. Oh happy day...
There was some of this in the garden last year - quite a bit, if I remember correctly - and it is lovely, especially in masses when the blue flowers on the long stalks sway in the breeze. But do I want it everywhere?
So I have a decision to make. I think I'm going to end up going with a non-perfectionist compromise: root it out completely of the herb beds on either side of the verandah stairs, keep it to a dull roar in the big central perennial beds, and let it do whatever the heck it wants to in the beds along the property line.
There. Decision made. Sounds good to me.