Blue tits preening on passionflower vine

This morning I've been watching a pair of blue tits which appear to be rubbing themselves on a passionflower vine that hangs over a pergola. At first I thought they were just being clumsy but it really looked like the motion of dustbathing! They were doing this, and then perching on a branch for a bit and preening.

Has anyone seen this before?

I know that passionflower leaves are toxic, is it possible this is some kind of parasite deterrent? I don't want to jump to conclusions but it seems really cool.