Could Delias blanca have spread between Borneo and the main Philippine islands by the southern route, from eastern Borneo through what is now known as the Sulu Archipelago to Mindanao (the movement could have been in either direction), rather than via Palawan?
But Delias blanca is known from Sabah (N. Borneo), not from Eastern Borneo. I doubt that a big Delias would have been missed in a highly collected island like Palawan though, same for Eastern Borneo. If the locals find singhapura there, they would have found blanca too.
It has a patchy (known) distribution in the Philippines; Luzon, Bohol, Negros, and Mindanao each have a known subspecies. Only those from Luzon and Mindanao are generally available; the others seem to be considerable rarities in collections. I'm guessing that it may occur on other islands too, its rarity and lack of collecting on those islands being the reason that other populations haven't yet been discovered. It's true that Palawan is much better collected; perhaps it occurs only in a small pocket that hasn't been explored, which is my supposition for the lack of recent specimens of the Borneo subspecies. And the same situation for the several rare Delias species from Biak, not having been seen since the days of the Pratts, to my way of thinking.