He joined Manchester United's youth academy in 2002 and worked his way through to the first-team, as well as spending time on loan to Blackpool, Birmingham City and Celtic.

He then played for Reading and Nottingham Forest and in Major League Soccer for FC Cincinnati before returning to English football with Rotherham United.

Blackett began his football career with youth club Fletcher Moss Rangers, and signed for Manchester United in 2002 at the age of 8.

[10] During the 2014–15 pre-season, Blackett played at centre-back for Manchester United in a 3–5–2 formation and his performances led to him earning praise from new manager Louis van Gaal.

In the club's final pre-season match against Valencia at Old Trafford, he provided the assist for Marouane Fellaini's stoppage-time winner to make the score 2–1.

[11] As a result of an injury crisis and the departure of experienced defenders Rio Ferdinand and Nemanja Vidić from the club, Blackett was drafted into the first team squad on a permanent basis.

[12] He also played in the next game away at Sunderland, which ended in a 1–1 draw with Blackett making an important clearance during the match to help earn Manchester United a point.

[26] On 8 March 2023, Blackett returned to England when he joined Championship club Rotherham United on a short-term contract until the end of the season.

[32] Blackett was recalled to the squad for the final Victory Shield match against Scotland in November 2009;[33] he replaced Sam Magri in the 69th minute as England won 2–1 at the Tynecastle Stadium in Edinburgh.

[44] Blackett earned his second call up for a friendly with Denmark in January 2013,[45] he started the game before being replaced by Lewis Baker in the 67th minute as England won 3–1 at the Keepmoat Stadium.