During the 2002 NRL season Blacklock was granted a release from his contract with the Dragons in order to play rugby union. Some speculate that this move was motivated by the disappointment Blacklock felt after not being selected for State of Origin, despite being the leading try scorer three years running. After a short stint playing rugby union with the New South Wales Waratahs, Blacklock returned to the Dragons during the latter half of the 2003 NRL season, where he again proved his worth by scoring 14 tries in 12 games. 2004 was an average year for Blacklock, where injuries kept him off the field for long periods of time. Blacklock returned to help the Dragons at the end of the year, which included scoring crucial tries in the Dragons' round 25 come from behind win against Manly, as well as their narrow loss to the Panthers during the first week of the finals. From 1997 to 2004, Nathan Blacklock played 137 first grade games for the Dragons, scoring 120 tries and 14 goals for a total of 508 points, giving Blacklock one of the best try scoring strike rates of any Rugby League player. In his entire playing career from 1995 to 2004, Blacklock played 142 career first grade games. During that time he scored 121 tries and kicked 14 goals for a total of 512 points. At the end of 2004, Nathan Blacklock left the NRL in order to play in the Super League for English team Hull F.C. In his first season with Hull F.C. he scored 22-tries in all competitions, including a hat-trick away to the Huddersfield Giants and at home to the Leigh Centurions. He played for Hull F.C. on the in the 2005 Challenge Cup Final victory against the Leeds Rhinos. In August 2006, it was announced that Blacklock would have to retire from all forms of football due to a chronic knee injury. Blacklock had been planning to retire from the game at the end of the season but when the extent of his injury was discovered, the club decided to release Blacklock from his contract as he was no longer able to play. After his retirement was announced, Blacklock came out and stated, "I'm disappointed I've had to call it a day, but I simply can't give the 100% I want to anymore because of injuries. I can't put my heart and soul into playing. Everyone at the club has been great to me and I wish the team all the best for the play-offs and the future." At the time of the news, Hull F.C. were sitting second on the Super League table and were a real chance to reach the Grand Final at Old Trafford for the first time in the Super League era.