Black Beauty's unexpected success? (4,5)
I believe the answer is:
dark horse
I believe this clue is a double definition.
'black beauty's' is the first definition.
(Black Beauty is a fictional black horse)
'unexpected success?' is the second definition.
(term for an unexpectedly successful competitor)
(Other definitions for dark horse that I've seen before include "Competitor whose abilities are unknown" , "Brown heroin" , "Little is known of its properties" , "Competitor about whose abilities little is known" , "Black Beauty, for instance" .)