I'm fine with the way blocking works. I don't think someone else should be able to determine what I can and can't see on site.

If the issue is escalating to offsite content, I doubt double blocking is going to impact much.

Double blocking in the forums was done away with because it was considered unfair so I don't see why the shoutbox would be any different. One of the main issues with the old system was that people still circumvented the block usually by gettin a party that had not been blocked to talk to the other party or circumvent it some other random way.

The forums in particular were done away with because people still wanted access to a forum post itself regardless of who was on the other end. I.e. Selling threads for instance... Because they might want to sell or advertise to others on the thread, but if the blocked person originated the thread, they would not be able to actually see the thread... Likewise for debate forum topics if they wanted to respond to others in the thread but could not because it was blocked.

I am assuming the same applies to the shoutbox.