The 26-year-old moves to The City Ground on a permanent contract having left Reading at the end of last season.
Blackett started his career in the youth ranks at Manchester United, making 12 first-team appearances for The Red Devils, and gained experience on loan at Blackpool, Birmingham and Celtic.
The versatile defender who is comfortable at left back and centre back, moved to Reading on a permanent basis in 2016 and played a key part in their run to the Championship play-off final in 2016-17.
The former England youth international made over 120 appearances for the Berkshire side but now joins up with Sabri Lamouchi’s Reds ahead of the 2020-21 campaign.