Blind joined United from Ajax last summer.
"Of course, we're proud," van der Sar told ManUtd.com. "Even though we had a little bit of a headache over how to replace a player like Daley.
"He was eight years old when he came to Ajax and I saw him as a little boy with his father [Danny] when he came to the matches and was in the players' lounge or at the training ground. It was great to see the way he developed in those 16 years through the Academy and into the first team, playing in a fantastic World Cup, and then to get a great transfer to a club that is very stable and a great club under the guidance of Louis [van Gaal]. I hope he continues with the way he is developing.
"He has really shown a lot of maturity in the way he plays, whether in midfield or the left-back position, of course everybody is really proud of how he's doing but there must also be compliments for the coaches at Ajax and our philosophy of bringing players through into the first team and making sure they get a great career afterwards."