The type of material you're probably thinking of is called PDLC film and I believe it's still quite expensive in the neighborhood of $50-100 / sq ft. Its default state is opaque and power must be continuously applied (around 1/2 W / sq ft) to make it transparent, which is the opposite of what is most convenient for most applications.
Edit: a few years ago I was working for a company with deep pockets, a brand new office building, and a penchant for cool stuff like this, and we had a sample panel for evaluation. Ended up passing because it went against (literally) the culture of transparency, and the cost was through the roof compared to blinds in the few places and times they were needed.