> For Slack's primary use-case of intra-organization communications, this is completely useless and wouldn't work anyway (if you block and hide messages from a team member, how are you going to collaborate?). If there is abuse, it should be up to management/HR to step in and address this issue.

Disagree. My employer's slack has about a million members. I don't collaborate with all of them. Speaking for myself, I would be glad of the opportunity to mute or block some of them.

HR's job is to protect the company, not the employee, so I would not expect them to help except in cases of extreme harassment. And even then, self-service tools that do not make you a target of further harassment are a foundation of public social networks. Why not private social networks too?