No. Such content is inappropriate and often not in line with healthy sexual values. It may also have a negative impact on one's mental state and perception of healthy relationships.
Blackmail has no justifiable relation to the inappropriate act of watching masturbation. Any form of blackmail is unethical and illegal. In a proper society, we should not have stories where these two are related in a positive way. We should uphold values of respect, privacy, and legality.
No, not in general. As long as they are private and not interfering with one's daily life or relationships, they are a normal part of human sexual expression.
No. Teaching masturbation stories is completely inappropriate and violates moral and ethical norms. It involves private and sexual topics that are not suitable for public discussion or teaching.
It depends on the context. In a private, consensual conversation among adult friends who are comfortable with the topic, it can be appropriate as it's a form of sharing and communication. But in a more public or professional setting, it's usually not appropriate.