Blocks are a learned behavior caused by the approach-avoidance conflict. You block on sounds you anticipate stuttering on because you don't want to stutter / are afraid of how it'll sound, but at the same time you want to say what you want to say. The might of wanting to speak hits the might of not wanting to stutter and you get stuck until either the fear relaxes and you can speak, you use a trick to escape, or you start making the sound you are blocking on. It all feels involuntary but it's a conditioned response you are doing.