It's a blunder because Qh4+ wins your bishop on c4 (9... Qh4+ 10. g3 Qxc4 -+), and you could have stopped checkmate without doing that and while developing/getting your king to safety by castling. If there are multiple ways to stop checkmate, and one of them loses a piece for no reason but the other one doesn't, the second way will be a better move. Both of the relevant lines are in this board analysis:
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How is this a blunder if I atleast prevented checkmate?