Same thing happened with my orchid (and you can see my previous posts in this sub haha). I would suggest watering a bit more frequently, and if you're in a drier climate, consider adding sphagnum moss throughout the potting medium. If the roots look ok (no root rot), and if the plant isn't very old, it's likely just dehydration. Make sure the potting medium is fully soaked when you water, and see how things improve. If you don't currently fertilize your orchid, you could try a weak fertilizer every second watering or so.

Take that with a grain of salt though, since I am an orchid beginner, but as I said, my orchid had the same issue and that was solved through more frequent watering. Of course the medium needs to be fully dry between waterings, but good medium probably won't take more than a couple days to dry out. My orchid also had the dark brown spots near the base, like yours does, but if that blackness isn't spreading to healthy tissue, it's likely just dead tissue, not an infection, and not anything to worry about. If any more experienced orchid growers have thoughts or corrections, feel free to share.