For my first SW tank I bought a domino damsel. Unbelieveably mean, it would pick on my poor little blenny who's only purpose in life is to scoop up sand in its mouth and sift through its gills... Now I wonder, why on earth buy a blue damsel, when you could get a friendly little blue/green chromis???
As far as catching the damsel, I say suspend a nice big clear plastic bag semi-buoyant in the aquarium, keeping it as open as possible. Then when the damsel swims in and gets stuck, just calmly reach in (with your hand approaching from the side the bag's open), and pull the bag out.
Works great....