If you can rub the spot off then it is sooty mold which often, but not always, grows on sap released from insects like scale. So check carefully for insects. If you find none, you may want to do a prophylactic spraying with oil (neem or horticultural).

Thanks Vladimir. It doesn't rub off like sooty mold. No other leaves look like that one. I'll do a neem spray next chance I get.