Potted mint w/spreading black spots/edges NOT rust
I just bought a small, healthy-looking mint plant at my local farmers market and put it in large pot after carefully pulling apart the roots, as shown by the farmer. It is in a bright, but not hot window (bright earlier in the day than later). I planted it in Miracle Gro Organic potting soil. It keeps getting large black and brown spots that seem to spread, as well as black edges. Then those leaves look like they are wilting. I'm trying to keep it moist and not too wet/dry.
I did some research and found photos of rust and it does not seem to be that disease.
I wonder if I am watering inappropriately? I tried to let it dry out (even though the farmer told me to keep it moist) but that didn't help. Then I've kept it moist, but that didn't help.
Weirdly enough, I noticed a similar problem on two leaves of my new sage plant also -- and that one is a few pots away from the mint. A coincidence?
I love herbs and would prefer to grow mint myself if possible. I live in a high-rise apartment building and cannot put it outside. We had mint growing practically wild in my backyard as a child and have read of people having good luck with it in a kitchen garden. Plus, I am good with plants, so hope I can figure this out. Many thanks.
I'm not sure how to post a photo, so I included a link to a badly-taken photo of the problem. (I'm not much of a photographer.)
Here is a link that might be useful: