One issue is that it will slow games down. If I have a chain of actions in my hand, ready to play, I'm not going to want to slap them down quickly, thus revealing to my opponents that may or may not have Blockades what they might choose to block. I'm going to play a Lab, wait to see if it gets blockaded, then play a Village, wait again, then play Margrave, wait again, and so on.
At present, long chains of Conspirators, Laboratories, and Grand Markets already take a while to slap down on the table, especially when you're playing your whole deck every turn, so often I'll just play three Labs at a time and take my total of +6 Cards in a single chunk. A reaction card like this means everybody has to slow down the playing of their turns -- ironically, even when no one has a Blockade in hand.
The mechanic is interesting otherwise, so I wonder if the effect might be achieved some other way. Maybe a targeted discard attack is effectively the same thing? Every other player reveals his hand and discards an action card that you choose. It's not quite the same, but just about. That's a very powerful attack, though -- Donald said he tested a card like that, and it didn't go over well with his testing group. But I don't know why a form of this idea couldn't work somehow.