The Seasiders have also acquired French trio Ludovic Sylvestre, 26, Elliot Grandin, 22, and Malaury Martin, 21.
Ex-Barcelona trainee Sylvestre joins from Czech side Mlada Boleslav, former Marseille player Grandin from CSKA Sofia and Martin was last with Monaco.
Cathcart never played for United's first team but spent time on loan at Royal Antwerp, Watford and Plymouth.
Good to finally get some players in but are we going to sign any strikers? Ted - Premier League Tangerine Midfielder Sylvestre has also played for Sparta Prague and has signed a two-year deal at Bloomfield Road with an option for a third year, as has winger Grandin.
Fellow midfielder Martin is a former French youth international and has signed a one-year deal after impressing on trial following his release by Monaco.
Blackpool's Premier League opener is at Wigan.