My search for the waterproof fountain pen ink lead me to the Noodler's black ink.
I find it strange that this ink is described as bulletproof. It has absolutely no meaning at all.
The glass bottle contains 3oz (90ml) of ink. That's a lot of ink. It's filled to the brim so be careful when you open it for the first time. The bottle is tall and I can imagine the difficulty of reaching the ink when it's less than half full.
This is a water based ink. Most of the reviews I've read are favorable on using this inside fountain pens. I'll update this review again in the future if it kills my pen.
I'm using this ink inside a Lamy Safari and it flows very smoothly.
The black is not completely dark black.
You can see from the hatch lines I drew above that the black black is actually at the tip of the stroke where my pen leaves the paper. If you require super black, you've to go with pigment ink which is not suitable for fountain pens.
So is this ink waterproof?
It depends on the paper you use, and how dry the ink is.
This is the ink on normal writing paper. The ink dries fast here. Seems to bind well with the paper. I applied a wash over it with a waterbrush and it doesn't smear. Note that the paper wraps from the water.
This ink doesn't seem to bind well on cartridge paper.
On the relatively less absorbent paper, the ink doesn't dry as fast. If you're just using it for writing, it doesn't really pose a problem. You have to let it dry totally for watercolour work.
The Noodler's ink is waterproof on writing paper, watercolour paper but strangely not so on cartridge paper.
It has minor quirks, but otherwise a good performing, value for money ink. Since it's water based, it's good for fountain pens as well.
This ink is made in USA, and looks like only Amazon US branch stocks it.