What are the qualities needed to be a #6, for you ? For me winning the ball and organizing the defense are the two most important.
A #8 is a all rounder, he is good at defending and attacking and he doesn't have to be your best ball winner, just has to be decent at it.
For me, box-to-box midfielders are the main ball winners because they have the stamina and aggression to do so. Since they can cover lots of ground, I expect them to be able to win the ball quickly with their aggression and strength.
Holding midfielders, on the other hand, are great at plugging in the gaps, cutting off passes, slowing down opposition attacks, forcing them out of dangerous areas, and filling in for defenders who are out of position. Here, I don't expect them to mainly be a ball winner, though being able to win the ball with tenacity and aggression is a great quality to have.
What happens, more often than not, is that box-to-box midfielders who aren't great going forward are then shaped as ball-winning midfielders who don't really get involved going forward. Here, I refer to Wanyama, Fernando Reges, McCarthy, Bentaleb, Matic, Schneiderlin, de Jong, Flamini, etc, today and Lass, Gattuso, van Bommel, etc., back then. Some of these midfielders then develop a few skills that a holding midfielder should have, which goes very well with their stamina, aggression, and tackling.
Blind doesn't have the aggression and strength that most of these ball winners have. There was a reason why he wasn't seen as a great left back at Ajax but a great, promising defensive midfielder there. It's why most posters talk about his lack of pace in midfield and the fact that he's weaker than most holding players. It's why Chadli was also causing him to be easily dragged out of position when he went to the left side. You wouldn't see Carrick, Busquets, or Lahm play like that and perform well.
If I see Blind as a #8, then I wouldn't see him as a great #8 when compared to players like Strootman, Nainggolan, Fernandinho, Khedira, Matuidi, etc. He may be quite good at going forward with the ball, but he's not good at winning the ball like the players I mentioned. However, as a #6, if he plays with more control, he'd be up there with the best defensive midfielders in the world.