There absolutely is a black market for selling airplane parts to sanctioned countries. As an example, Cuban state airline Cubana has been flying ATR turboprops with American made engines (and many other parts) since 2002 or so. Even with wide sanctions, Iran has maintained an operational fleet of old Boeings, and during past decade even newer aircraft.
https://www.aerotime.aero/articles/31395-here-is-how-iran-sm...