This may sound stupid, but how do you double unobtainium?
Supercharging the blaze burner doesn't seem to be melting my unobtainium, so how do you do it?

It doesn't have a molten version. Best you can do to increase the yield is by crushing, which has a 20% or 55% chance of an extra crushed unobtainium. In 1.0.8, I've adjusted the recipe for crushing unobtainium ore block to give two crushed unobtainium with a 55% chance of a third.