Why did you word that as if the poster didn't create the content within the blog?
Many comments here are pointing out Fetlife as a refuge for some tumblr ex-pats. Speaking as a Fetlife user, it's not hard to see why the 'their' is in quotes. When you see a tumblr profile on someone's fetlife profile it's often described by the profile owner as "you can see the kind of stuff I'm into on my tumblr" with a small ratio of original content to reshared content. Either images from a movie, online porn, or just "reblog" of another tumblr user's content when not the blogger's own images that are being posted.
This isn't me taking a position on adult content on tumblr, denouncing it or condoning it either way-but I think that bit of context might help answer your question.
It doesn't define the entire experience for the entirety of the porographically inclined userbase, but it's relevant.
To me, "Content" implies stuff you uploaded directly, being the source yourself.
I can see the confusion now.
You'll sometimes see someone tag a picture post, if they tag it at all, if it's something the blogger actually created themselves with #oc for "original content" or they'll just tell you as much in the caption. Not always, but sometimes.
I'm not on tumblr for the porn (I read it for the articles, I swear), but a lot of the mechanisms and meta-memes of how people use the platform are pretty consistent across subgroups.