He was slow to reach peak fitness in 1948, and was unplaced in The Derby, but showed his best form in autumn when he beat a strong field in the St. Leger Stakes.
Because of the rules imposed by the Jersey Act of 1913, Black Tarquin, like many American racehorses, was not eligible for inclusion in the General Stud Book, and was therefore technically a half-bred rather than a Thoroughbred.
Standing 16.3 hands high, Black Tarquin was an unusually large and heavily built Thoroughbred, which meant that he had to be trained with great care.
In August, Black Tarquin reversed the form with Birthday Greetings to win the prestigious Gimcrack Stakes at York in a new track record time of 1:10.3 for six furlongs.
[10] Although he did not run again that year, as Boyd-Rochfort wanted to give the horse more time to develop,[11] Black Tarquin's performances established him as one of the season's leading juveniles.
[11] In 1948, Black Tarquin was aimed at the 2000 Guineas but sustained a bruised heel when training on unusually firm turf and missed the race.
At Epsom, Black Tarquin started at odds of 100/7 for the Derby in a field of thirty-two runners and was reported to be strongly fancied by his trainer.
[16] Black Tarquin did not race again until the St Leger at Doncaster on 11 September, in which he was ridden by the Australian jockey Edgar Britt and started at odds of 15/2.
Doug Smith attempted to make all the running on the outsider Alycidon, but Britt guided Black Tarquin to the lead in the last quarter mile and won comfortably by one and a half lengths.
In the straight, Edgar Britt moved Black Tarquin up to challenge the leader and drew level a furlong from the finish, but Alycidon pulled away again in the closing stages to win by five lengths.