Grievances are the legitimate means of enforcing the contract. Grievances are not acts of violence, they pose no threat to any member of management, thus they are not a form intimidation. Now, if you had proof that said steward were making threats of physical violence, that would be intimidation and coercion, and if he were using compromising photos of members of management to get them to do something against their will, that would be an example of blackmail.
You are upset that the steward is doing the one thing he is supposed to do, and mischaracterizing it as something illegal or unethical. You clearly have no clue how union/management relations work, neither do you understand employment laws, nor criminal laws for that matter.
Don't join the union, that's your right, but don't let your lack of understanding cause you to slander unions or the steward who is just doing his duty.