Witch Blood Mage Ascendency Passive (Open Sores)

I am using the Blasphemy buff skill with a socketed Despair curse to always curse enemies that are near me, I thought the Open Sores passive would make it to where they would be cursed always no matter where I was as long as I got into range at least once. It does not work the way I thought and the curse disappears off of them when they get out of range of my aura. I am looking for a good explanation for this please as it is kinda frustrating to figure that out. Thank you for your time!