To edit your blacklist, navigate to My Account >> Settings. Type in the tags you don't want to see in the 'Blacklisted Tags' box. For example:
yaoi nude male_focus
Put each blacklist on a separate line. If tags are placed on the same line, only posts that contain all of those tags will be hidden.
In addition to blacklisting based on normal tags, you can also blacklist posts by pool ID number (e.g. pool:1234), rating (e.g. rating:e), uploader (e.g. user:bob), status (e.g. status:flagged), and score (e.g. score:<0).
When your blacklist is active, a category called "Blacklist" will appear on the top of sidebar on the left. You may click on a blacklist entry there to temporarily disable it and show the posts that match.
The following blacklist is automatically set for anonymous users and new users accounts:
spoilers guro scat furry -rating:s
Note that this only applies to user accounts created August 6th, 2015 or later. Accounts created before that had no default blacklist.