Just waking up to this. What a tragic disaster.
Have to anticipate the Blackhawk pilot had no idea was in approach vector for DCA.
The one thing I am struggling with is the post of flight path. (@DavidM ) as it appears the chopper turns directly into path of plane. Directionally, he was headed toward plane, meaning the landing lights would/should have been visible. Unless they meshed with lights from surroundings
DCA final approach has unusually late and steep turns due to airspace restrIctions. I bet it ends up being a factor. The chopper pilot requested visual separation and saw the CRJ. From that point he must have been disoriented.