Why is this move a blunder?
After Nxd4 your threat against the rook is no longer available.
after exd4, white will play dxc6, gaining a knight.
it also ruins your position if you take the pawn back on c6.
But hardly a blunder? Or what am I missing. Mistake perhaps...
Giving away a minor piece (bishop or knight) is definitely a blunder.
borovicka75's analysis is correct. After White plays 11. c3 ..., you lose a minor piece. Since your Knight is being attacked by the d5 pawn, it should have been moved to a safe square instead, such as 10 .... Ne7 or 10 .... Nd4.
After Nxd4 your threat against the rook is no longer available.
after exd4, white will play dxc6, gaining a knight.
it also ruins your position if you take the pawn back on c6.
In your variation, Black would play Nxd4 in response to Nxd4, not exd4. Nxd4 would be a blunder for White.