Unfortunately I disagree with the others. It looks just like one of mine did and everyone said they thought it was nothing to worry about... however I never did work out what it was before it killed the plant.
It was like a dry necrosis spot on the side of the plant. It didn't spread fast but did over about 6 months go from about 5mm diameter to about 10mm diameter. More though it just kept going deeper into the plant and steadily the leaves dropped with it as it did.
I'm afraid I can't help because I never did discover the cause, but I would keep this plant away from your others so it can't spread to them. I hope I'm wrong and this is something different, or I hope someone can help with how to treat it, but personally that black spot doesn't look good to me, sorry