WoWDB

 Only available for purchase while  Darkmoon Faire is active since the vendor is located on Darkmoon Island - unless you are willing to pay the gold to buy it from the Auction House.

The  Inky Blackness buff it applies to the player has a duration of two hours, so you definitely get your gold's worth out of it.  The screen effect creates the appearance of a darker "night" than normal.  If you are like me and keep the CVar ffxGlow disabled, do not worry - the effect still works.  The effectiveness of the screen effect does appear to be affected by what world environment visual effects are present, however, so Your Mileage May Vary.

It does not persist through death.