Blue Girl (1899–1919) was an American Thoroughbred racemare that was the Champion 2 and 3-year old female in 1901 and 1902, respectively.

Blue Girl was bred in Kentucky by the Ezekiel Clay & Catesby Woodford breeding partnership and foaled at Clay's Runnymeade Stud.^[1] She was sired by Sir Dixon, the 1888 Belmont Stakes winner, out of the mare Bonnie Blue.^[2] Bonnie Blue was sired by the influential American sire Hindoo and also produced the semi-successful stallion Blues. Blue Girl was sold as a 2-year-old in 1901 to John E. Madden, the owner of the Lexington stud farm Hamburg Place.^[1]

Blue Girl was trained by John Madden as a two-year-old and won the Juvenile Stakes, Eclipse, Great Trial, and Great American Stakes for Madden, netting $38,230 in purse money. She was bought by William Collins Whitney in late 1901 and won the Great Filly Stakes winning $23,975. As a 3-year-old, Blue Girl won the Gazelle and Ladies Handicap. She started in the Flying Handicap, run at Sheepshead Bay, but she became lame during the race.^[1] This was her last start, and overall Blue Girl started 12 times and won 7 races.^[3]

Blue Girl was retired in 1903 and was sent to Whitney's Brookdale Stud farm. She was sold to Frederick Johnson (as representative for Harry Payne Whitney) for $10,000 in October 1904 after William Whitney's death.^[4] Blue Girl was sent to Britain in 1912, but returned to the United States in 1915^[5] due to anti-American Thoroughbred sentiment and the passage of the Jersey Act. She produced thirteen foals, but none were as successful as Blue Girl. Her offspring include:^[6]

• Tammany Hall, chestnut gelding by Meddler (1904)
• Blue Grass, chestnut filly by Hamburg (1906)
• Dalenburg, chestnut colt by Hamburg (1907) (exported to Britain)
• Bay filly by Hamburg (1908)
• Eton Blue, bay filly by Hamburg (1909)
• Brush By, colt by Broomstick (1910)
• Delft, bay filly by Burgomaster (1911, died 1926), granddam of 1926 2-year-old champion Scapa Flow
• Blume (GB), chestnut filly by Broomstick (1913)
• Bit of Blue (GB), chestnut filly by Lemberg (1914)
• Cobalt, bay gelding by Willonyx (1915)
• Blue Laddie, bay gelding by Cylgad (1916)
• Sky Blue, chestnut filly by All Gold (1918)^[7]
• Chestnut colt by Pennant^[8] (1919, died 1920)

Blue Girl died in 1919 at the Brookdale Stud.^[7]