It's actually not derogatory: "A dark horse is a little-known person or thing that emerges to prominence, especially in a competition of some sort, or a contest

It's actually not derogatory: "A dark horse is a little-known person or thing that emerges to prominence, especially in a competition of some sort, or a contestant that seems unlikely to succeed." https://en.wikipedia.org/wiki/Dark_horse https://en.wikipedia.org/wiki/Dark_horse