As far as E-Ink goes, my understanding is that it works by moving charged ink particles from one end of a microscopic capsule to the other, so that white and black particles switch places, resulting in a visible color change. All this is to say that the display surface isn't exactly changing color or opacity, it's just moving it around, so I doubt it could be used to create a surface that goes from transparent to opaque and vice versa.
On the other hand, LCD could be used to create an opacity-changing window, but LCD panels block a lot of light even when they're fully "transparent," which is why they consume so much power (their backlights have to be very bright).