Blind only had a significant impact in the last two Eredivisie seasons, and the time he spend as a holding midfielder was one season. He was *really* bad in his first season as left back and even after showing some improvement in his second year his contact would probably not have been extended had Boilesen not been injured.
Vernon Anita was the key player that reinvented the DM position and helped Ajax win the first two of four championships, and in honesty was the better holding midfielder (except he made a dumb transfer to Newcastle and is now rotting away there as that club has no clue how to use him). De Boer is blind as a bat when it comes to judging midfielders and messed about with failed experiments like Janssen before injuries forced him to play a technically gifted player like Aissati and later Anita in that position, which proved crucial in their second season half comeback. After Anita left De Boer mucked about with that turd from Liverpool (Poulsen) when injuries yet again rescued him as he was forced to move Blind to DM and again the team rapidly improved. Blind only made the world cup squad as Holland has no half decent left backs, and I doubt Blind would have made a transfer without his WC performance and Van Gaal. Blind made the most of the chances he got, but he can't complain about luck in his career.