I'm pretty certain that Mont Blanc make there own cartridges - my Mont Blanc pens are designed to take Mont Blanc cartridges, - which is what I use most of the time when travelling - and they certainly make their own ink (in bottles).
Mont Blanc cartridges are international cartridges. They will work in any international cartridge pen(I have used their cartridges on occasion).
I like Mont Blanc inks as a whole, and IMO they have one of the best current production bottle designs out there. Montblanc Midnight Blue(blue-black) is my primary ink, and I keep the bottle sitting on my desk because it looks so darn good.
With that said, Montblanc is on my "not nice" list now. For quite a while, the formulae of the bottled and cartridge Midnight Blues were different. Specifically, the bottled Midnight Blue WAS an iron gall ink. Iron gall inks are very traditional inks that actually oxidize and darken on exposure to air. They are effectively permanent. All iron gall inks have a dye component to them now(generally a dark blue) as the iron gall is colorless on initial application. The dye can be partially washed away with water, but the iron gall component remains behind.
I have used quite a few iron gall inks, and they tend to be fairly dry as a whole. Midnight Blue is different-it is quite lubricating and overall is one of the nicest writing inks in this category. It goes down as a very dark purple blue color that over the course of a week or so will turn a very nice blue-black color.
About two years ago, Mont Blanc started using the same ink formula for Midnight Blue bottles as they'd been using in their cartridges. It's not a bad ink, but is quite different from the old Midnight Blue.
As I watch my old bottle drop perilously low, I've been desperately searching for a bottle of the old stuff. In fact, at one point I found a seller on Amazon who listed it under the old p/n and confirmed it when I asked. I bought four bottles(over $80) only to have bottles of the new p/n show up. Needless to say, they went back.
I've since been watching Ebay hoping some bottles of the old stuff will turn up. Although I don't have any MB pens, they had a faithful ink customer in me because-as I said-it's in my go to, every day, general purpose pen.