This only looks at the effects on Alice and on Bob, as a simplification. But with blackmail “carrying out the threat” means telling other people information about Bob, and that is often useful for those other people.
When the public interest motivates the release of private info, it’s called ‘whistleblowing’ and is* legally protected and considered far more moral than blackmail. I think that contrast is helpful to understanding why that’s not enough to make blackmail moral.
*in some jurisdictions, restrictions may apply, see your local legal code for a full list of terms & conditions.
I think you’re right that it’s not trivially negative sum because it can have positive outcomes for third parties. Still expect a world of legal blackmail to be worse.
This is in response to other writers, esp. Robin Hanson. That’s why.