Canon FTb viewfinder stays black after exposure unless lens is loosened

This is most likely a camera body issue, not a setting. Based on the reported fix, the mirror had slipped slightly out of position and was sitting askew. That can cause the reflex mirror to catch on the rear of the mounted lens as it tries to return, leaving the viewfinder black. Loosening or removing the lens gives the mirror enough clearance to drop back into place.

If the camera works normally without a lens, that strongly points to mirror interference rather than a shutter problem. In this case, gently reseating the mirror restored normal operation.

Because this is a mechanical film SLR, a shifted mirror may indicate aging adhesive, wear, or the need for service. If the problem returns, the camera likely needs cleaning/adjustment or repair rather than any lens-setting change.