The 403 was the planned response when you tried to DM any remote user, because federation of message was just not implemented. With v1.7.0 we implemented it and you will not get a 403 anymore
Think of it this way: there is currently no way to block an instance from a users perspective, there simple isn't (and yes we should add one). What blocking a domain is for: I hate everything the guardian posts, I don't want to see anything talking about that.
Do you mean blocking a domain? So navigating to /d/domain_to:block? If you do: that block all posts that have a url linking to kbin. Since comments do not link to anything they will not be blocked by this. If you want a feature like this, please consider making an issue over on github in the mbin repo