1. It is B3 2. Look at game report and stockfish moves before that move. 3. Look at chess principles (no need to do it all the time). No need to take me if you take this advise because it works in all games.
How is this a blunder?

1. It is B3 2. Look at game report and stockfish moves before that move. 3. Look at chess principles (no need to do it all the time). No need to take me if you take this advise because it works in all games.
Ah yeah it is B3; typo on my part. Thanks for the advice! What do you mean by stockfish moves before that move? I know stockfish is the engine but before that move everything is rated as "best."
Heya! First post here so sorry if I get some procedure wrong. Also quite new to chess so excuse my ineptitude. Anyhow, played a game and thought it was pretty fun but noticed the engine marked one of my moves as a blunder. I can't seem to figure out why though. Anyone see something I'm not?
My "blundered" move was Knight captures Pawn on B3 (edit: typo; used to say D3):
Later I got my queen into the back due to a blunder from black and won. Black resigned and otherwise (quite obviously) would have been checkmated shortly:
Here is the whole game:
Bd7 9. Bxa8 Qxa8 10. Nxb3 c6 11. Qf3 c5 12. Qxa8+ 1-0