Hi, Gothichome. I'm not sure what to make of it. The unstained area is about 12'x12'. This particular room and the room behind it (now, the kitchen) was the first addition to the original house. The wall that the two [mismatched] doorways are on might not have always been where it currently is. It's such an early addition, I doubt the kitchen as it is today is anywhere near what it was then -- if it even had one when the addition was completed. There's a chimney on the outer wall that has no corresponding mantle on the interior anymore; it's just an empty hallway that now leads to the downstairs bathroom. (In fact, I have no fireplaces anymore.) All that said, the black outline on the unstained area could have been anything. I know so little about the house as it was in those early days when the additions and changes were taking place; I've only been able to speculate based upon what I've observed in the actual construction of the additions. The wood floor I uncovered in that room appears to run under the hallway, kitchen, and downstairs bathroom, so I'm at least confident this was all constructed during the same time frame. You can also see the 'seams' in the brick on the exterior where the additions begin and end. The downstairs bedroom, which is behind the kitchen, was yet another addition.
I do like the reddish color on the small area that's actually painted.