My house is early 50’s cinderblock construction. The interior side of the blocks were furred out with 3/4” thick strips and finished in plaster (uninisulated). At some point, the original owner put furring strips over the finished plaster wall in the living room and hung 60’s/70’s era paneling. I took the paneling down & had cellulose blown in behind the original plaster wall, then installed 3/4” thick styrofoam panels over the original plaster, and between the newer furring strips. Then I hung 1/2” drywall where the paneling was, primed and painted it in a medium light green. It took a few years, but a clear outline of every furring strip began to appear as dark shaded areas, along with perfectly round darker spots... which were the heads of every single countersunk & spackled drywall screw I installed. Anybody hear of this??? What should I do??
My personal bet is that the cinder block is wicking water, which is wicking through the furring into the drywall. That will absolutely cause the issues you're describing.
My bet is there's no vapor barrier. You said there was no insulation and in-wall insulation is typically faced with tar paper, which is typically your vapor barrier.
If you're looking to fix this there's no easy way around tearing it down. You can leave the furring up, but I would add something like 6mil plastic over it before applying new drywall. You might also want to add some waterproofing membrane to the cinder block, to minimize the wicking.