Most controllers have a poor understanding of how difficult it is to visually acquire and avoid traffic. Radar controllers don’t see aircraft at all, of course, and in the tower cab, you get to know exactly where to look to correlate the picture on your radar display with what you’re seeing outside - and then get to subconsciously thinking it must be that easy and fast for the pilots as well.
“The CRJ on final for 33 out of (altitude)” may’ve been better, but nighttime visual separation below 10,000 just doesn’t work, imo. Few people outside of DCA certified controllers are qualified to offer a fully informed opinion, but personally I would not have been comfortable issuing that clearance. Day or night.