Not the main favored candidate. Someone somewhat unkown who exceeds expectations of all others in an event. This saying is said
to go back to the Victorian politician Benjamin
Disraeli who, apart from
finding time to regularly become Prime Minister, also found time to write a novel
called "The Young Duke". In the book there
is a description of a horse race in which
the two favourites cannot make the running. In the meanwhile "a dark horse...rushed past
the grandstand in a sweeping triumph".