Blue Flame is somewhat specific of a skill. It’s best for units who lack atk, def, and res, so that they can’t hit super hard and they also have a hard time finding a stat on which to scale their special. Rutger is actually one of the better candidates for this skill. The catch is that if you use wrath, he maybe needs to figure out a way to control the amount of damage he takes, because he doesn’t have desperation and his atk stat is a bit low to be playing for OHKOs. If you boost his defense into the upper 30s or even to 40+, then by that point he can probably switch to bonfire and save you some rare fodder (unless you still want to give him wrath).
The other use is for units on defense teams with WoM, usually pre-charged using pulses.