some have poker faces I have blunder face. If my face light up, start looking for the mistake.
do chess players realize when they blunder
With me, it varies widely. Sometimes as I'm moving, but my reaction time isn't fast enough for me to not let go of the piece. Sometimes just after I let go of the piece. Sometimes not at all until my opponent lowers the boom on me.
What's really depressing is when you both miss the blunder. When I was just starting out, I was playing someone near my own pathetic rating. In the middlegame, I made a move, not seeing that it opened me up to a backrank mate-in-2. My opponent made his move, then said, "Oh sh&#!". (He actually said this loud enough for everyone in the room to hear - It was a smallish local tournament.) I figured something was up, so I scanned the board but saw nothing. So we continued with the game. (By sheer luck, my next move removed the backrank mate threat; I still didn't see the threat.) Later at home, I ran the game through an engine, and it was only then that I realized how bad of a player I really was. 
Sometimes, it is good fun to fake a blunder.
Let go of the piece, invoke a horrified look on you face, slap yourself on the head and exclaim: "Oh...shit!"
That happened to me once. My opponent faked a blunder and i ended up exchanging my queen with a rook.
It's usually as soon as I let go of the piece. Twice it's happened to me in ONLINE GAMES!! As soon as I hit "Submit," I saw the horrible horrible thing I had done. Nothing to do then but sit and imagine my gleeful opponent at the other end saying, "Honey, come over here and look at what this idiot from the USA just did."
is it usually right away after they make their move or right after the opponet makes his move and then you realize... you messed up