Several months ago I encountered the same bug with prometheus-smokeping-prober, and fixed it, but my brain is a bit foggy right now as to what the root cause was.
However, for prometheus-blackbox-exporter at least, the --log.level flag is respected so long as you build / run it with the dependency versions specified by go.mod - up until using prometheus/common v0.26.0 or later - at which point the --log.level flag is no longer honoured. This version number sticks in my mind from the smokeping_prober bug too. I will follow up with more information shortly.
