[FEN "r2q2nr/p3kppp/3bp3/2p2b2/Bn1P4/1P2PN2/4NPPP/R1BQ1RK1 b - - 0 13"]
I dont see whats wrong in b3 I am making some space to bring black bishop into game attacking nb4
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I wouldn't call it a blunder, as White still has a large advantage after 1.b3. But more efficient is 1.dxc5 Bxc5 2.Bd2. Here White's attacking the knight, but has only spent one tempo developing his bishop. Your idea with 1.b3, 2.Ba3 would take two tempi.
In this kind of position where the opponent's king is vulnerable, each tempo matters more. You want to take advantage of things before the king gets to safety.