He was a warden. Frequently in the company of other wardens. It's their job, and one of the reasons Harry didn't want to become one. Too many secrets, and that's for a good guy. I agree with you at the peabody thing. I didn't think of it that way.
We don't know when Justin went bad, though -- he might not have turned to dark magic until after he left the Wardens.
Thinking about it, what we know of his attempts to enthrall Harry and Elaine may well indicate he wasn't very well versed in it -- he needed Elaine's help to subdue Harry, he had to take them one at a time and planned to restrain Harry physically... and even his enthrallment of Elaine apparently wasn't total, since she sneakily used a binding spell she knew Harry could break.
So in sum, I don't think we can use Justin as an example until or unless we have a better idea of just how often he actually
used black magic.