So everyone can understand, and avoid speculation.
As a reminder, I'm a USAF vet with ATC experience, a licensed pilot and scored perfect on the ATC entrance exam in 1992, only to have my invitation / job offer rescinded when GHWB froze ATC hiring upon taking office.
Setup: AA5342 was on an ILS approach to DCA Runway 1. At the outer marker he asked ATC for a visual approach / circle to land on R33. This is common. The CRJ can land shorter than larger aircraft (R1 is ~6900' long, R33 is 5200' long and the turnoff places them closer to their arrival gate) so the request is not unusual. ATC has the authority to either grant or deny.
PAT25 (the SH-60 Blackhawk) was headed south alongside the Potomac and requested clearance to transit the airport's Class C airspace on a course that would intersect the approaches for both R1 and R33. That clearance was granted.
It should be noted that the 2 events - AA5342 deviating from its approach to R1 and PAT25 transiting the airspace across the approach path - would have created a distraction for the approach controller by invoking 2 events that, while not unexpected, were exceptional with regard to the normal traffic flow.
The controller asked PAT25 if he had visually acquired the CRJ. The helo pilot responded in the affirmative so the controller issued an instruction for the SH-60 to pass behind the jet.
This would have been perfectly fine if PAT25 had, in fact, acquired a visual target that actually was AA5342. In the visual confusion of all the lights in the area, it's clear that what he thought was a CRJ on approach to R33 was, in fact, something completely different.
The resulting error (failure to "see and avoid") on the part of the PAT25 crew resulted in the collision.
Ultimately the NTSB will likely conclude that the "see and avoid" failure was determinant.
However, some logical blame also lies with the controller. With multiple aircraft on final approach, he could easily have refused PAT25's initial request with the instruction to "avoid the Class C". Additionally, rather than pass responsibility for separation to the helo pilot - at night, in congested airspace, with multiple converging targets, he could have "handled" the SH-60 by instructing it to make a shallow left turn which would have put it, with 100% certainty, behind and underneath the CRJ.
Objectively, this is just a tragedy. But all aviation tragedies are rooted in human error, at some point along the way. We should insist that federal authorities work to ensure that procedures - and training - are grounded in common sense and logic and that traffic flow around airports like DCA (which is, at all times, an accident waiting to happen) is closely examined and appropriate remediation steps are taken.