Hard to say from one blurry photo, but the black spots are probably the honeydew (excrement) from some small sap-sucking pest that is growing sooty mold of some kind on it. That could be from aphids or any number of similar pests. Or it could be thrip poo.

You could also try spraying it vigorously with a strong jet of water on both top and bottom surfaces of leaves and pruning off damaged areas to see whether the plant recovers. Mint is very resilient. I generally don't like to spray stuff on herbs because the leaves are intended for consumption. But if things don't improve, you could try insecticidal soap.