I dont believe there are any saturated niche when blogging, take your desired one and using SEO keyword suggestion tools create few nice targeted categories on your blog, put it in your title and do updates and trades all the time using those specific keywords from title as an anchor.
You might find saturation when targeting main keywords like teens, milfs, lesbians, big tits.. as with every other form of promotion. As of submissions I get on Jerking Resources, the most blogs are shemale niche, but many of them are from my partners, since I'm doing that the most.
Hope it helps.
