They've been fairly solid so far, but I don't particularly think they've received a stern test at all yet. We largely controlled the Spurs and Villa games, even if we only one goal, and were also the more dominant team against Newcastle despite our failure to score. They've not had a game yet where they've regularly been up against it. City have also comfortably won all of their games, but have definitely come up against more tricky opposition than us.
Our tougher games will definitely be a massive indicator as to Blind's ability at centre back, though. He's looked comfortable there so far, but it's going to be a lot more difficult for him if he comes up against a tricky striker who's constantly pressing him and has a lot of physicality to his game.