Megan
Redgiant Unmult is one of the Best way to remove black bg.
But make sure your flare size is not more than the Layer Size as it will make boundaries which will create problem even if you remove black bg.
Best way is to make a new comp with large size, add flare to it and then remove black bg.
Vishesh Arora
VFX and Motion Graphics Artist
Films Rajendra
Blog:
https://digieffects.wordpress.com
Demo Reel(3D):
https://www.youtube.com/watch?v=lHPgIJU_BR8