My blocking (and probably yours) is caused by a lot of tension in my body. My speech therapist (who I have been going to for the last year) has pointed out that when you block- you do in fact tense up all throughout your abdomen, diaphragm, throat, which is stopping the air from being released. When I recognized this pattern of tension, I also started to notice how I would breathe VERY deeply to compensate for the shock of not breathing. This is really unnecessary, and makes the block even more of a shock to the system- making it hard to create natural speech (nobody inhales so deeply like they have been under water for the last 5 minutes before they speak). What worked for me was becoming aware of the tension, and slowly relaxing my body, and in turn not over-inhaling when trying to say the word again.