That is what we discovered on Wikipedia relating to the The Darkish Horse:

A dark horse is a previously lesser-known person, team or other entity that emerges to prominence in a situation, especially in a competition involving multiple rivals, that is unlikely to succeed but has a fighting chance, unlike the underdog who is expected to lose.

The term comes from horse racing and horse betting jargon for any new but promising horse. It has since found usage mostly in other sports, sports betting, and sports journalism and to lesser extent in nascent business environments, such as experimental technology and startup companies.