Blocks are moments of stuttering where no speech is produced. It can feel like the air is stuck and you are unable to move forward with speech.
Personally, I find that using prolonged speech helps with my speech blocks a lot. I avoided learning prolonged speech for a long time. I reasoned that as I was mostly experiencing blocks on the very first sound of a sentence, it would be of no use to try and prolong my speech - as I wasn't producing any sound that I could prolong. Curiously, the act of deliberately producing the whole sentence with prolonged speech results in less issues with blocking on the first sound.
The key is to deliberately prolong the whole phrase, not just the difficult sound or word. So, instead of "Eeeexcuse me" you would say "Eeexxxcccuuussseee mmmeee".
The way I explain this is to imagine the brain as a computer and our speech as a computer programme. Imagine blocks or other moments of stuttering as errors or crashes as a result of running a computer programme with high processing demands. When we block we are not only having difficulty with the first sound of the sentence - it is the complexity of producing the whole sentence (the whole computer programme) that we are having trouble with. We could say prolonged speech simplifies the amount of information that the computer has to process at one time, resulting in smoother running.