I haven't used steam that much, and I barely installed it, but I'm guessing it automatically changed the folder is saves screenshots to? It could have experienced a problem and it could have done so.
steam has it's own screenshot system
it has nothing to do with changing blockland's
I find Alt-Key or Ctrl-Key helps to make sure it doesn't happen, my screenshot is set to Ctrl-/ and overlay to Alt-F3 and I have no issue