OK, I have found what it occurs to me as the most likely reason: a combination of technical issues dealing with gall and ferro-gallic inks on fountain pens and knibs, extensive chemical testing and research which ended on official instructions from several governments, including Prussia (the famous Prussian blue) , UK and United States. Then market adapted to this, based on new discoveries like aniline and synthetic French ultramarine
This can be found in a book from 1904 by David Carvalho, Forty centuries of Ink or A chronological narrative concerning ink and its backgrounds
It explains in great detail the whole process to obtain inks, how different inks where needed for pens and much more, a really interesting lecture. But even more important: UXers will be shocked as how this author researched based on nowadays common UX techniques.... more than 100 years ago. Furthermore: the reasoning behind the inks selection was made based on user needs, with a wide array of user cases
For those interested on the OP's question, you can skip directly to around page 150 and further
As a curiosity...
A little known fact…Charles Dickens increasingly wrote in Prussian
Blue ink towards the end of his writing career. His famous last novel
the Mystery of Edwin Drood is almost entirely written in Prussian Blue
Disclaimer: I obviously didn't read the whole book!