Explanation of Blocker bet
Blocker bets are small bets designed to aid the player in reaching showdown cheaply. They are mainly used in the hope that villain will be willing to call a small bet rather than make a big bet when checked to.
How to Use Blocker bet as Part of Your Poker Strategy
For the most part blocker bets are a psychological trick and have no actual grounding in poker theory (although many players will debate this). A small bet might even potentially have the opposite effect where it induces villain to raise where he otherwise would have checked back if checked to.
Making a small bet OOP might often still be a decent play, since it allows the bettor to extract thin value without over-investing. Even if the play is decent however, many players end up making it for the wrong reasons, since they are doing it in an attempt to manipulate villain, not to extract thin value. Of course, perhaps blocking bets do have the desired effect against individual opponents, but it would have to be analysed on a case by case basis.