Originally Posted by ATCBob

When it's busy, as a controller I always want to know when I was blocked so I can immediately re-transmit instead of wasting time waiting for a readback that will never come. When it's busy we need the regular rhythm of instruction/readback/instruction/readback to keep things moving. When we give "instruction" and get a readback of "blocked" we go back to "instruction" and the flow keeps going. If we give "instruction" and hear silence, now the rhythm is messed up and we have to decide to go against the flow with another instruction (when we're expecting a readback), that is likely to get blocked again if the pilot was just delayed in their response.

I've never heard other controllers rant about "blocked" either, although I've only worked in Center where about 95% of the pilots we talk to are professionals who can tell when it's the controller being blocked and not two aircraft blocking each other (when "blocked" wouldn't be appropriate). Maybe it's different in the approach and tower environment though.