To edit your blacklist, navigate to My Account >> Settings. Type in the tags you don't want to see in the 'Blacklisted Tags' box. Use a new line to create multiple blacklists. If tags are placed on the same line, only posts that contain all of those tags will be hidden. For example, "nude male_focus
vomit" will hide all posts that are tagged with vomit, and all posts that are tagged with both nude and male_focus. Post that contain nude but not male_focus will still appear, and vice versa.
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).
You can also whitelist tags within a blacklist. For example, "spoilers -touhou" will blacklist all posts with the spoilers tag, except for those that also have the touhou tag.
Wildcards are not allowed with blacklists.
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
Note that this only applies to user accounts created August 6th, 2015 or later. Accounts created before that had no default blacklist.