The only problem I have with the "Roberts was blackmailed" theory is that it is difficult for me to conceive of anything worth getting blackmailed over in Washington. We've had a horndog president getting oral sex in the White House, a D.C. mayor caught snorting coke (and now they want to build a statue of him), a congressman whose boyfriend ran a rent-boy operation out of his apartment, and so forth. Unless Roberts was filmed killing and eating a baby, I don't see what they can have on him.
The whole blackmail thing to me falls apart on the notion that material capable of blackmailing the Chief Justice into changing his vote on a single (if incredibly important) issue wouldn’t be used instead to pressure him to resign.
Its not like Roberts hasnt been fairly center-right on most of the other issues before the court. Why settle flipping him for a single victory on Obamacare when Obama could get appoint a fifth Liberal Justice as well as a new Liberal Chief Justice?
Occams Razor applies, IMHO. Roberts was either concerned about the non-partisan appearance of The Court, or wanted to teach the American public a lesson about the consequences of their electoral choices.
There is the possibility that his adoption of his children from Ireland was illegal.