Can someone please guide why c5 for black was a bad move here?

I do not trust computers for early position evaluations too, but certainly they are preferrable to the very shallow evaluation you've just made. You will never, ever learn something regarding the quality of XYZ move by using raw, unfiltered statistical data.

To the O.P:

- Why do you played your bishop at b7 at a very early stage?

Obviously, to get control over the central light squares (d5,e4)

So the right follow-up is adding more forces to control these 2 squares (...Nf6, ...e6 etc).

- Does ...c7-c5 stop d2-d4?

Obviously not, and in some cases there is the danger of that pawn advancing to d5, and limiting the b7 bishop for some time. If the pawn was sitting back at c7, he could probably undermine it by ...c6 and such.

I wouldn't pay attention to the computer calling any move an inaccuracy. It hasn't much of a clue. However, c5 isn't a great move either. e6 brings about the English defence, paying more attention to piece development. The problem with ...c5 is that you're now in a Symmetrical English Opening, very heavily analysed indeed.  White can play d4 and d5, and black's b7B isn't very good. I think that would be the natural approach and if white is a good positional player then should get an advantage. c5 is in itself a normal move but the addition of b6 and Bb7 seems to make it very passive if white plays well.

Hi manoj05, do you have Chessbase 15?

I think ...c5 is an error. As explained, white can build a superior position. The reason is that the B is misplaced on b7 .... it's a waste of a move if c5 is played because white can block the diagonal and black can't prevent it without getting a poor position. The result is that white is a move ahead in a position that's already slightly better for white. After ....c5, black cannot attack the pawn on d5  effectively.

It's not a great move, but neither is it so bad that white has something to be excited about. I'd call it one of those 2nd rate, but up to personal taste, moves.

I do not trust computers for early position evaluations too, but certainly they are preferrable to the very shallow evaluation you've just made. You will never, ever learn something regarding the quality of XYZ move by using raw, unfiltered statistical data.

To the O.P:

- Why do you played your bishop at b7 at a very early stage?

Obviously, to get control over the central light squares (d5,e4)

So the right follow-up is adding more forces to control these 2 squares (...Nf6, ...e6 etc).

- Does ...c7-c5 stop d2-d4?

Obviously not, and in some cases there is the danger of that pawn advancing to d5, and limiting the b7 bishop for some time. If the pawn was sitting back at c7, he could probably undermine it by ...c6 and such.

Not really, it's only that I play d4 openings all the time now because opponents playing black make a lot of errors early in the game compared with their better knowledge of e4 openings. And I know that it only takes a small mistake like that one in the opening and if white plays it right, black never gets much of a chance. It's usually safe for white to castle kingside and wait for black to castle and then throw the K-side pawns forward or make a piece sacrifice, because typically when black's bishop is on b7 it can take several moves to get into the game and meanwhile white can often break open black's king's position.

It's the opposite of white's strategy in the Classical variation of the King's Indian Defence, like the Mar del Plata where black is playing a piece down in the ending for several moves because of his bad dark-squared bishop on g7, so white plays to smash open the queenside, sometimes by sacrificing a piece and trying to queen a pawn in the ending when most of the pieces are off and black has less pieces to defend with. But in the game in this thread with a bishop on b7 and white's attack on the kingside, white would play to keep the pieces on because black's pieces tend to get in his way too much and white uses the extra space he has to maneuver into an attack. Really, I'm only just starting to properly understand these ideas  and it's sad my competitive days seem mainly over because the local chess clubs and leagues in the UK have mainly collapsed.

Exactly. You're likely to get a very bad Benoni after d4-d5.

It would also be incorrect to call it a blunder. ...c5 is not "sub-optimal", if you follow the explanation I gave. So early in a game, anything that doesn't lose or give the opponent a big advantage is simply an opening variation. If it were inaccurate then every opening variation that is currently out of favour would be "inaccurate".