Can a Blackguard suppress and re-activate Aura of Despair?

[Shin Okada]

And what kind of action are they?

A Blackguard is disguising and does not want to reveal it's true nature.

 

[shilsen]

The DMG doesn't say that it can be suppressed, but you can rule that it is similar to the paladin's Aura of Courage, which does not aid nearby allies unless the paladin chooses to. In that case, as a supernatural ability, the Aura of Despair would take a standard action to activate or deactivate.