An unexpected person who comes from behind to win at something.
It originally referred to horse betting. A horse with very poor odds of winning a race who would actually end up winning was called a dark horse. If someone bet on this horse they would end up with a lot of money, because it was thought the horse was unlikely to win. This is now used to talk about a person who comes from behind to win. It can be used in business to refer to someone who wasn't thought to have the highest sales rating, earning the highest sales rating or something similar.