Why is this a blunder?

I'm trying to figure out why moving my bishop to h5 is considered a blunder. Checkmate came two moves later...

http://en.lichess.org/jYfjLqDs#35

Your advice is appreciated!

18. ..Nxf5 and black has equalized an almost lost / lost position (the engine says it in the comments to his move)

See: http://en.lichess.org/EVAbpNBkscS6

This is Stockfish vs. Stockfish... it would be cool to have a feature that plays the engine against itself in a position instead of having to do it manually... but in this variation, black wins a piece.

Sorry, I meant http://en.lichess.org/KDpVpiJlkaP8

So, basically I just got lucky then? ;-)