Originally Posted by Pelion
The prerequisites for a blocking bet are:
1) You have a semi-decent hand that is worth seeing a showdown with.
2) Your hand is not good enough to call a big bet with even though you have a reasonable chance to win the hand.
3)Your opponent will only raise you with hands that beat you (i.e. you can safely fold if he raises).
If all 3 are true, then a blocking bet is a way of pricing your showdown. If any of these are not true then it is better to just check/call or check/fold IMO.