catchers blocking the home plate

Question
What is the official ruling for catchers blocking home plate?

Answer
Ken, great question!

Since you didn't mention which organization, I'm going to assume ASA.

Obstruction - Obstruction is the act of a defensive team member: A.  Who hinders or impedes a batter from striking at or hitting a pitched ball.  B.  A fielder, who impedes the progress of a runner or batter-runner who is legally running the bases unless the fielder is:
1.  in possession of the ball.
2.  in the act of fielding a batted ball.
Note:  Contact is not necessary to impede the progress of the runner.

As you read, in ASA softball you can not block a base until you have possession of the ball.  You MUST have possession of the ball, not waiting for the ball to get to the catcher, but possession of the ball before they can attempt to block a base.  This goes for every base, every time.  A defensive person can not hinder or impede the progress of a runner without possession of the ball.  

The umpire mechanic for this is to signal a delayed dead ball signal.  After time is called, award the runner the base or run to which they are entitled.  

However, there is one rule that will trump the obstruction rule, and that is an offensive player crashing into a defensive player.  Rules 8.7 Q States the following under the heading, "The runner is out":  When a defensive player has the ball and the runner remains upright and crashes into the defensive player. (The runner is out, if the act is flagrant, the runner may be ejected.)

So there you have it, the "block the plate" rule.  Now getting the umpire to enforce this rule may be more difficult as obstruction is not typically enforced as often as it should be.

Thanks for the question!