Does black really have a disadvantage? Most people say chess is a draw, and it is quite believable, white is but a tempo up, and skill plays a lot of part in the game. But even the most skillful agree that blunders are made, in some games more than other. 

How have blunders affected black losses? Have games where black lost been only played best or second best moves?

How many games, out of the ones that white won, ended because of blunders, and could have been saved otherwise? Is there a statistic for this?
We have no way of knowing this, chess isn't remotely close to being solved. We only know that as opening theory digs deeper, it finds it harder and harder to find advantages for white, so the game is probably a draw. But that's no proof.

If the game is a draw, then any game that is won by either side must involve at least 1 wrong move. But we don't know which move was wrong. It is likely that most games involve quite a few bad moves. Mine do, anyway.

Using a computer doesn't help -- they're nowhere near strong enough. A match between the two strongest computers on really strong hardware gives many decisive results, so they're not playing perfectly, and they look much deeper than 10 ply. Their scores (+2, -2, etc) are merely estimates and not the final truth.

White does have a practical advantage, proven by the fact that white consistently scores a little bit better than black on all levels. Apparently, white gets positions that are easier to play right.