What is "blue gown"

Blue Gown (1865 – November 25, 1880) was a British Thoroughbred racehorse that was the winner of the 1868 Epsom Derby and Ascot Gold Cup. He was one of the best colts of his generation at two, three years and four of age, but his form declined in 1870 after an unsuccessful period in France. He was retired to stud, where he had considerable success as a sire of winners in Germany. Blue Gown died in 1880 while being shipped to the United States.