How to put the phrase "dark horse" into use? For example, is Liu Xiang a dark horse?
The dark horse is someone people don't know about who surprises everyone by winning a competition or is all of a sudden successful in something. So therefore, use the dark horse where there is the surprise element.
Liu Xiang is not a dark horse. He was. Back when he won the bronze medal at the Paris World Athletic Championship in 2003, he might have been considered a dark horse by the pale faces (so American Indians called Europeans) because thitherto nobody had expected a yellow face to compete for top honors in the short track. The 110-meter hurdles dash has traditionally been a European and American sport dominated by whites and blacks. Liu is a mere exception.
At present, however, Liu is no longer a dark horse. He's a proven winner having bagged both Olympic and World Championship gold medals. He's also the world record holder. He's now the head of the pack, so to speak. He's the man to beat. In fact, it isn't too far-fetched to suggest that another Olympic Gold next year in this very city is his to lose. It will take a dark horse of the darkest color to upset Liu, judging from the reigning champion's form, professionalism and how he takes care of his body.
By the way, Ladji Doucoure, a Frenchman, and Dayron Robles, from Cuba, are also young and on the up. Either may upset Liu in Beijing but then both of these young men are proven winners. Doucoure has won a world title of his own and Robles beat both the Frenchman and Liu in an IAAF Golden League meet in Paris in July. In short, neither can be considered a dark horse even if they do sneak up on Liu next year. Personally, though, I don't see anyone spring a surprise on Liu next year. Not in Beijing.
Anyways, use the dark horse where there is the surprise element. Even then, use it sparingly. The dark horse is a cliché, i.e. something old. Old tricks don't always work as well as at first. Even gold, silver and bronze lose luster over time and so if you have anything original to say, say it or stay simple – a surprise winner is good enough in place of any dark horse.