KB3 - BLUNDER?

Which move? Move 33. Kb5 isn't a blunder according to my game report. It's a "good" move when Stockfish claims Re5 was best (Re7 also good).

I wouldn't worry about small centipawn differences when it misses the big picture. At that position, black is slightly better because of their one pawn material advantage in this endgame, so I'd look earlier if you want to find improvements which decided the game.

The main improvement probably would come from 29. Ke3 instead of Re1? because that's where you lost your pawn. As a good guideline, you also don't usually want to block a central pawn with a bishop such as 5. Be3 with a pawn on e2. You'll usually be better defending d4 with pawn to e3, or maybe advancing the f3 knight to e5. In this game, the computer didn't give 5. Be3 the mistake icon (?) because of advanced positional reasons involving the pawn structure of the fianchetto (with g3 and Bg2 planned), but that's more like 2000+ concepts, or at least 1800+ for sure. Keeping things more simple, you'll usually want to avoid blocking a central pawn with a piece (especially bishops).