301. And the clever go free 2012-1-20

The city jail was overcrowded. A judge ordered the jailer to move three prisoners to the nearby state prison. After consulting with the prison's warden, the jailer told the judge that the prison could take only two of his prisoners.

The judge said, "Then you'll have to release a prisoner. You decide which, and I don't care how you decide."

The jailer picked three prisoners at random, and had his guards blindfold them and bring them to the jail parking lot, strapped into wheel chairs, unable to see behind them.

He said to them, still blindfolded, "A guard is standing behind each of the three of you. Two of them, one of them, or none of them is blond. The others have black hair. I will remove your blindfolds, one of you at a time. If you can tell your guard's hair color, I will release you from jail to freedom by lunchtime. Otherwise, I will transfer you to state prison."

A guard whispered to the jailer.

The jailer said, "Smith, I hear that you are blind, so you don't have a chance to win this. We'll take you back to your cell."

Smith said, "I'd like to try, if I may go third."

The jailer said, "Okay." He walked over to Jones and removed his blindfold."

Jones looked in Smith's direction, then toward the other prisoner, Baker, then said, "I don't know my guard's hair color."

The jailer said, "Too bad." He walked over to Baker and removed his blindfold. "Soon, Baker said, "I don't know my guard's hair color."

The jailer was silent for a moment. He hadn't selected one for release and two for transfer. Before he could give another order, Smith said, I know my guard's hair color, and I can explain how.

What was his guard's hair color, and how did he know—being blind, and all?

Answer

The hair color for guards of Jones,  Baker, and Smith could be any of the following:
Jones    Baker    Smith
black    black    black
black    black    blond
black    blond    black
black    blond    blond
blond    black    black
blond    black    blond
blond    blond    black.

Jones looked at Baker and Smith, but couldn't tell, so rule out
black    blond    blond.

Baker looked at Jones and Smith, but couldn't tell, so rule out
blond    black    blond.
Also, rule out
black    black    blond,
because if Jones didn't see
black    blond    blond
and Baker saw
black    ????     blond,
then Baker would have known that his guard had black hair, so rule out
black    black    blond.
So, Smith's guard doesn't have blond hair, and must have black hair.