Blind returns to Ajax for a club record fee.
"First of all I want to say how proud I am to be a part of the Manchester United family," Blind wrote on social media.
"To play for the biggest club in the world with such a history meant a lot to me. I had tour great years with four trophies. I want to thank everyone, the managers, coaches, the medical department, everyone who helped me to settle in so quick at the beginning, of course my teammates and last but definitely not least the fans.
"I want to thank you for all the love and support I received and still get. I will never forget some of the special games and moments, I will you the Theatre of Dreams! But now, it's time to go home."