The global Blacklist gives you the possibility to use one Blacklist for multiple WordPress Blogs. You need to setup a place where you store your Blacklist (e.g. Webspace, Dropbox, etc. - but HTTP only) and put it into the Field "Update URL". How you Built up your Blacklist (e.g. PHP-Script with Database, simple Textfile, etc.) is up to, but you need to make sure, your Update URL returns one value per line seperated by "\n". Put the Update URL in all your Blogs where you want your Blacklist, and setup the update rotation according to your needs. The global Blacklist will be activated by adding an Update URL. The global Blacklist is case-insensitive.