I wouldn't agree that *most* blue-black inks are IG inks. I have several BB inks and none of them are IG inks. I would never knowingly use an IG ink in any of my pens.
As there are now ways of producing permanent inks without the use of IG, I personally can't understand why modern day fountain pen users still want to put old recipe inks containing what are essentially wood particles into their nice pens. I'm sure there are some who will have their own reasons for doing that. However, there was probably a good reason why many ink manufacturers changed their ink recipes and excluded IG as an ingredient.