Thanks, Tim, for forwarding further information on behavior from the observer. For me this bit of the description is rather compelling:
"The wing coloration does not look quite right for the bird I saw but this is as close to what I saw as I have been able to find. It paused in flight right in front of us to grab an insect in flight, almost like a flycatcher or a bluebird."
as wagtails can be quite agile in flight, and can often be seen hawking insects from perches in their breeding habitat.
I looked this up in Alström and Mild's "Pipits and Wagtails" just to make sure I'm not over-inflating my own recollections from Sweden/Finland. In their species account for White Wagtail (which includes Black-backed as a subspecies, lugens) they note on p. 386,"Also captures insects in the air after fluttering chases."