I doubt anyone would consider it blackmail in the legal sense. But I'm putting myself in the owner's place: upfront contract, month 4 of 6, more equity promised upon receiving a deliverable. Losing the developer is a major setback for my startup, and the developer threatens me with creating this setback unless I give a large amount of equity. I would certainly feel that the developer is trying to take advantage of me at that point.

Like I said, it's one thing to negotiate better compensation, or to walk away for a better offer, etc. It's another to say "I can screw you over and I will use that to get more from you".

And like I also said, there are two sides to every story. This is seen exclusively from the owner's side.