Originally posted by Murfy And is something deteriorating inside that caused the back sticky mess in the first place?
The blob is what was left of the mirror bumper foam. Some bumper materials crumble with age and exposure to ozone. Others turn to a tarry, sticky mess. In that case the tar often gets flung onto the focus screen. Sorry this happened to you.
It is usually a good idea to evaluate both the mirror bumper and light seals on vintage cameras before or shortly after purchase.* They are often bad. Replacement is part of a standard CLA as would be cleaning of the focus screen. Your screen is likely cleanable, but not without removing it from the camera (requires partial disassembly). The cost for a CLA would be in the $45-$75 range and is generally worth it for a fully mechanical camera like the K1000.
Steve
* test by touching a toothpick to the foam. If it is not resilient, it should be replaced.