Thanks, Simon. Your answer makes sense, though I expect there's something a bit more complex going on.
I posted this question on the Adobe Lightroom forum as well and got a couple of answers from Jeff Schewe.
He first responded that "The -20 Blacks that result in a blue channel clip is actually to be expected. And while it's correct that the Whites and the Blacks settings "manage clipping" that's not to say you should NEVER clip, just be aware of it :~) Sometimes intentional clipping helps an image..."
I then asked what Lightroom is optimizing for and he answered: "It's optimizing the black point according to Thomas Knoll's opinion of what a black point should be. It's not always 100% correct, more like 80/20 and sometimes does something a bit less than optimal, but it usually is in the correct direction. From the result, season to your own personal taste :~)"
Here's a link to the full thread:
https://forums.adobe.com/message/9478134#9478134I've attached screen shots of the image he's referencing (before and after screen shots, zoomed in to show the areas where clipping is indicated -- these are the shots I forgot to attach in an earlier post).
I think I've gotten about as far as I need to on this. I'll keep experimenting with it because what's happening in the mid-tones is interesting.
I appreciate your help.