Browsing the web, I discovered a fascinating piece on the discovery of Prussian Blue on About.com/painting. You can read the article here.
I should say that Prussian Blue (and its more modern counterpart, "Thalo" or Pthalocyanine Blue) are very strong, staining pigments, not to be used in large quantities. They will overwhelm any color with which they are mixed, and they will overwhelm any painting if used in any quantity.
That said, Prussian or Thalo Blue is a warm blue which, mixed with lemon or zinc yellow, makes a bright, jewel-like green. I include it on my recommended limited palette because of that, but it should be used sparingly and with caution.