Flight Instructor reported a NMAC with a helicopter which flew through the airport traffic pattern. Instructor took evasive action to avoid a collision.

Narrative

Reporting pilot was performing left traffic pattern work at ZZZ Runway XX. While on the crosswind; a Blackhawk training flight approached the airport environment from the east tracking northwest on 'Road X'. ZZZ Tower provided traffic advisories to which both parties complied and confirmed visual contact. However; as the Cessna continued downwind; the helicopter's route seemingly converged towards the downwind to base leg at pattern altitude. Helicopter then descended about 100-200 ft below pattern altitude and banked west bound into the base leg environment of the traffic pattern and ultimately continued west. In doing so; the helicopter pilot essentially joined the traffic pattern environment; cut off the Cessna and proceeded on an extended base leg flying west away from the airport environment. Cessna diverted from standard pattern to make an evasive sharp right climbing turn east to avoid rotor wash. This report is to point out how close the helicopter on the aforementioned training route got to the base to final leg of the airport environment. Despite confirming positive contact; no apparent actions on the helicopter's part were made to remain clear of the airport traffic pattern environment.