Even if it was electrostatics, the moment the hanging blocks touch the sitting blocks the charge would dissipate and the electrostatic effect would vanish (the hanging blocks would move away). But that's not the case. They stay next to the sitting blocks.
Discussion
God damnit man, watch the discussion. It's at 1hr 20 min. Even the glober arguing for it is more clued up on this than you are.
I didn't make it to the discussion about electrostatics on the video, it's just too long. If you can point to a specific minute, I can analyze it.
I won't argue that electrostatics and gravity are related. They are, and that's why a field called "electrogravitics" exists. But that has nothing to do with the Cavendish experiment.