Tragic.
TCAS (for collision avoidance) was switched off on both aircraft, which is standard practice below 500 feet. The reason for that is part of the avoidance process may be an instruction to descend, and that’s a dangerous strategy at low altitude.
Equally, ‘visual separation’ of air traffic is an unwise process at night, even in good weather with no other visibility restrictions. Especially in areas with busy lighting on the ground. The onus is on the pilots (of both aircraft) to maintain separation by “see and avoid”, sometimes with guidance from ATC which is only ‘advisory’ in the US.
In a number of jurisdictions outside the US, and for some non-US airlines wherever they’re operating, it’s not a permitted procedure at night. The AA-5432 Bombadier CRJ-700 which crashed, in common with many airliners, does not have a cockpit windshield arrangement that’s conducive to maintaining visual separation.
AA-5432 has been on approach to Runway 1, but was asked if it could switch to Runway 33, to which the pilot agreed. About 1.5 miles ahead of AA-5342 was another aircraft of exactly the same type on approach to Runway 1 and behind it was an Airbus A319 on the same approach.
The instruction by ATC for the Black Hawk to “pass behind” AA-5432 was potentially problematic in the circumstances, given that the helicopter crew were probably expecting to see a single Bombadier heading for Runway 1. They may not have been aware that it was now heading for Runway 33, or that there was a second Bombadier ahead of it still using the approach to Runway 1. In that case, they may well have flown behind the first Bombadier to avoid a collision, but straight into the path of the second one.
Military flights in the US (including a Black Hawk on a training mission) must coordinate with ATC and can only legally cross a major approach pass to an airport if explicitly cleared by ATC, or conducting an authorized emergency/security operation, with ATC aware.