Blocking isn't a "type of stutter", it's an avoidance behavior, similar to all the other things we do to not stutter. Blocking is just a more intense avoidance. You want to say something but when it comes to a word or sound you're going to stutter on, you really don't want to show stuttering so you stop yourself. It feels involuntary but it's a learned behavior. This is good because it means it can also be I learned.