Carrick is the most intelligent. He's a matured Blind.
I'm not sure on this. They both show lots of intelligence, but I would say they both excel in different areas.
Blind's ability to offer himself for a pass and to be in space is better than Carrick. By some distance I think. The games against Chelsea and City where he was on his own in midfield. Compare those to Carrick against Arsenal, who whilst being good defensively was almost seemingly hiding from the ball. However, I think Carrick spreads the play better, moves it into the attackers better, knows when to play the ball low and hard or when to have it hold and hang.
I also think Blinds covering of space and sniffing out potential danger is better. A moment against Chelsea really amplifies this notion. Hazard carried the ball forward on the left, our whole squad got dragged across, he brought it inside and switched it to Willian. Whilst the rest of our squad was busy turning and shuffling back up the pitch desperately, Blind was there, waiting.. he won the ball immediately, danger snuffed before it had really begun. Again, another contrast - N'Zonzi goal for Stoke where Carrick gets dragged towards Bojan/Assaidi after Fellaini had lost it. We ended up with 3 players ball chasing pretty much, and an N'Zonzi standing completely free in the middle on the edge of our box.
I think Blind is better at pressing. Multiple times a game we'll see him pushing someone back towards their own goal. Not once yet do I remember him getting overturned as a result, because he seems to pick his moments very well. When other players do, one ball later and they've taken themselves out of the game and are desperately trying to recover. Not Blind.. not yet anyway.
Carrick is more likely to put in a last ditch tackle or block. He's less likely to get dribbled as he screens well but always keeps himself in contention. I don't think he gets taken on quite as much. Not that Blind does much either, but more.
Looking at the stats, Blind averages 2.8 tackles and 3 interception per game this season. Carrick 1.8 and 1.5 respectively. To caveat this, Blind has mostly played in the diamond formation with 4 at the back, and Carrick 5 at the back. If we take Carricks from last season, it's 2.1 and 2.8. Season before 2.3 and 2.1. Both lower, but again admittedly, he wasn't quite 'alone' (though it seemed that way) in the midfield so had less opportunity to make tackles/interceptions presumably.
I appreciate that I'm trying to paint a picture of generic strengths over another using individual moments. But whilst those moments stick with me and lend as a good comparison, I wouldn't say that I let them dictate my over-arching thoughts.
For the most part, they are completely interchangeable. They outshade eachother in different areas but they are accomplished at absolutely everything required for the role.