However, I do 100% agree that "Next" and "Previous" buttons should never be target="_blank". That's a pretty unnatural and terrible experience. Only chapter links and external references should be.
At the end of each individual post, there's a link to see the next one. It's very much the same concept as your next/previous buttons, which you agreed shouldn't be opened in a new window. That's primarily what I take issue with.
Personally I'd also argue that the TOC shouldn't be new windows either, that the user should have to take explicit action to follow a link in a non-standard way. But reasonable people can disagree about the merits of this in certain cases.
Sharlin's comment below also touches on this.