"Fond" is a French word meaning in that use "background" of the dial.
So basically "fond blanc" means white background dial.
"Porcelaine" is the name of a material, translated in English by "china", used for making dishes.
I guess it means that it is a white dial looking like the aspect of a white china dish surface.
In my older PP catalogs the 3520D is not indicated with a "Fond blanc porcelaine" or "cadran blanc porcelaine". References 3919 or 3802 had a "Cadran blanc porcelaine" in the catalog.
3520D is written to have a "Cadran blanc laque" meaning a white lacquered dial.
I guess the names "porcelaine" or "lacquered" where used to describe the result and not the process, if you follow me, so the use may have changed in time depending on who directed the catalog.