Yjere has to be a partial obstruction in your second floor drains, and the sink with the blackup is probably the most downstream drain on the second floor system…provided you mean that the sink backs up when it’s not be used while other fixtures are draining. If, instead, you mean that the sink backs up when IT is being used, then you have a simply clog in that sink’s drain.
Thank you NtP. Makes sense. The sink fills from the drain when not in use at a rate of about 1/2 gallon every 2 hours. This sink was previously slow to drain and clogged 2 days ago and I used Drano crystals (has worked well before in other homes). Drano and plunger have not released the drain and resulted in the added bonus of rising water. Am I right to assume the obstruction you mentioned is closest to the clogged sink or is it possible that the obstruction is located at the other 2nd floor bathroom? Of note, the water coming in from the drain appears to be fresh, clean water. No smell or discoloration. I also cannot tie an event (running water in sink/tub, toilet flush, etc.) to the rising drain water. It does not suddenly “fill up”, but rises so slowly as to not be visible to the eye when observing for several minutes.
Here’s a strategy that should work. Remove the sink trap and clean it out. Then snake out the drain behind the sink wall. Then snake out the drains from above by snaking out through the vent. If you have cleanout plugs, snake through these also. That should clear any blockages.