Typically black spots on the outside of Napa cabbage won’t affect the inside leaves, but this depends on when the cabbage was harvested and how many days it has been in cold storage. Black spots can occur during growth or due to mold. It is best to remove the outside leaves and discard them, then wash the remaining cabbage before use. If the spots are on the inside leaves as well, return the cabbage to the store for a refund. Unless your grocer knows of these problems, they can’t identify the best produce suppliers or problems in their own storage/handling.