Basil is native to India, so you can understand why it struggles to grow when the weather turns colder in other climates. Basil likes to grow in full sun and in a humid environment where possible.
When black spots appear on basil, people tend to assume it’s from a fungal infection. But while fungal infections do happen, they are nowhere near as common as frost damage.
The easiest way to tell if your basil has been damaged by frost or cold weather is to look at the topmost leaves. These are the youngest leaves on the plant and are therefore the most vulnerable to cold.
If the majority of the black spots are found on the uppermost stems, then it’s most likely due to cold damage. The cold may have even killed off the top set of leaves completely.
Black spots caused by cold damage will start at the tips and outside edges of the leaves. They will not form randomly in the middle of the leaf.