Hullo!
Blackguard's Aura of Despair has no visual or text -indication whether the feat has been activated or not, or when the aura duration ends. Mayby a function that tells on the combat-log or otherwise, or even a pop-up that the aura has been activated and when it is deactivated after the pre-determined duration has run down.
There are similar spells and effects I'm sure, but for spells the effect itself disappearing can be noticed easily from character-sheet if you're observant. Some spells have a magical "poof" -visual indicating the spell-effect has ended which can be noticed clearly if you're aware of your surroundings and character.
For Aura of Despair, only way to know if it's active or not is when you approach an enemy in melee combat when the combat log informs you that some or all of the nearby enemies in proximity are under the effect of the aura.
'Course you can just re-activate the aura mid-combat, which is perhaps a teeny bit of hassle to do or have extra focus in the middle of combat .. But, I guess this is just a point of user-friendliness, in my opinion of course.