The site would die of course. To save it they would attempt to get rid of the block feature but if everyone blocked each other then clearly everyone hates each other and is unlikely to keep using the site
I think A would kill the site - Remove the block would not be a good idea leaving aside people who would leave it would be creeper paradise maybe on reflection go back to A and make it a site of blogs is the only way it would survive as a safe forum. I have seen other forums that are antagonistic and they are not fun to be on which is what B would be like.
My blocking list is empty. I support (and can/will answer) contradiction. The aim of blocking is to protect girls from PUAs who send pics and angry people who send hate pms. A guy or girl able to defend himself or herself doesn't need it.