[doppelganger]

I think they have their place, generally when you have a decent but not great hand and you suspect your villain of the same.

Think of it this way, let's say you're in position on the river with a medium strength hand like bottom 2 pair on a 3 flush board. As the hand was played through the turn you suspected your villain may have completed the flush but aren't sure. If not, you think your hand is most likely good.

Villain checks. What do you do here? Unless he's a tricky villain this generally means that he missed the flush or isn't confident in his hand, so you can value bet for 3/4 of the pot (or bigger if you think you might need some FE).

Now let's say instead of checking, villain puts out a bet slightly less than 1/2 pot. Now it could just be a blocker, but it could be the flush trying to get value without scaring us away. What do we do? We call. If villain has a similar strength hand that we beat, he just got to set the price for showdown. If he hadn't, we may have cost him an extra few BB's for a hand he would still need to call with.

I think if you're going to add blocking bets to your repertoire, then you probably need to balance them by occasionally making similar bets with monster hands looking for value. If you keep the frequency similar they'll never know how to respond and you can save yourself money when playing marginal hands OOP that you still want to showdown.