By volume, it is probably the majority of ink sold.
Several countries require children to use fountain pens at school, or still have widespreaf use even if it's no longer officially required. Germany, the UK, I think India.
The blue ink that everyone uses most of the time is erasable with a chemical eraser pen [1], which everyone in my class (in the UK) owned.
Other colours were not erasable, so my gothic rebellion (requiring me to write in black or red) meant I had to be correct first time.
[1] https://en.m.wikipedia.org/wiki/Ink_eraser § Chemical