dawn on (someone), to

An unexpected potential winner. The term dates from the nineteenth century and comes from racing, where a horse is termed “dark” when its ancestry and history are unknown. It was so used by Benjamin Disraeli in his novel, The Young Duke (1831), but the precise origin is obscure. Some think it comes from the owner’s dyeing a horse’s hair to disguise it and so get better odds; others cite the practice of a particular American horse trader who made his fast black stallion look like an ordinary saddle horse, rode into town, set up a race, and consistently came out a winner. The term was soon transferred to political candidates on both sides of the Atlantic. The first American presidential dark horse was James Polk, who won the 1844 Democratic nomination only on the eighth ballot and went on to become president.