1. What does the 'Blacklist option when unsubscribing' feature do?

Unsubscribe the user from all mailings.

Unsubscribe the user from the mailings he previously chose.

Sends a request from the user to be unsubscribed.

None of the above.

When testing the feature it seems that the feature allow to propose the unsubscribe link in the mail and when clicked it allow you to choose which campaign you want to unsubscribe from.

So for me 'None of the above' or when clicked the 'Unsubscribe the user from the mailings he previously chose.' but never 'Unsubscribe the user from all mailings.'

Hope it helps.