“Adult” or mature content blogs are allowed but pornography is not allowed.
From the TOS http://en.wordpress.com/tos/
•Post explicit sexual materials that can be considered pornographic, such as explicit images or video of sexual acts or close-up images of genitalia;
•Post links or banner ads to adult-oriented affiliate networks, such as pornography site signups;
•Post links, text, or images promoting or advertising escort services;
•Post images of extreme violence or gore without associated context or commentary;
•Post images of child pornography;
•Post content that promotes pedophilia, such as blogs with galleries of images of children where the images, content surrounding the images, or the intent of the blog is sexually suggestive.