If he cannot commit diablerie, then his card text does not force him to: he is not "stuck", he is free to take any other legal action.
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Phillipe Rigaud
G4Sabbat. Black Hand: Phillipe cannot block undirected actions. When any older vampires are in torpor that Phillipe may diablerize, he must attempt a diablerie action (unless he must hunt).
Rulings
As long as there is an opportunity for diablerie, he must attempt it, even if he did so already and unlocked.
Black Hand is not a title, it is a trait unrelated to any sect. The trait is not lost if the vampire changes sect.