There is no automatic setting you can use and no warning page. You go to the front of your blog and you hover over your avatar on the far left hand side. There a dropdown menu appears and your click “report as Mature”. Then Staff will mark your blog accordingly. More information on the impact here > http://en.support.wordpress.com/mature-content/
Thank you TT, That helped. I have tried to put, in the beginning of my more (possibly) offensive writings, a notation of possible offensive material in the folloing blog but wasn’t sure if it, alone, was sufficient.