The Bic Cristal is my all-time favourite pen, to the point where I don’t really understand why other pens exist. I was recently thinking of getting a Mont Blanc for fancy business writing (contract signing), but went with the Cristal. Not kidding.
You mean a Mont Blanc ballpoint pen? Because I wouldn't use fountain pens with their water-based inks for signing contracts (even inks that claim to be waterproof).
Water based inks come in tons of flavors. You can have washables (most of your royal blues), "permanents" (most blue blacks and special formulations) which leave a faint mark you can't remove (GvFC inks sans Royal Blue, Pelikan 4001 Blue Black, etc.), pigments which cling to your paper like their life depend on it or you can go full send and get "cellulose reactive" inks which will not leave your paper unless you destruct the said paper.
Many "permanent" inks on permanent rollerballs and gels (e.g. Uni SuperInk) is also water based, yet they don't and won't leave your paper, because they're made of UV resistant pigments.
Right, or you can use an oil based ink with a good ball point pen and not have to worry about where to find "cellulose reactive" inks and how they will interact with your fountain pen.
Noodler's inks are dime a gallon, esp. in the US, and they're guaranteed to be pH neutral in most used colors (bottles state if they pH neutral), so they don't damage any pen, modern and vintage.
The one flaw: no upside-down writing.