The 20-year-old has started in all three of United's Premier League games so far this season, and has admitted that he is slowly getting used to the new system used by manager Louis van Gaal.
"You have to be disciplined with it and be able to criticise yourself," Blackett told the Daily Mail.
"It's not an easy thing to do and can be quite difficult, especially if you are going to watch a game where you know you have not played your best.
"But I feel it is something you have to do if you want to improve. It is especially helpful when you are trying to learn how to play a new system [playing three at the back] like we are at United now."
United face Queens Park Rangers when the Premier League returns next week.