Waterproof Fountain Pen Ink

[Zaknafain]

I'm searching for a waterproof fountainpen Ink that does hold up to watercolorwashes (there are a couple of so called "waterproof" Inks that don't).
It should be very black and most important it should have a good flow.

I've heard about Noodlers Polarblack. Does anyone have experience with it?
I already tried Noodlers Heart of Darkness which is supposed to be waterproof. Its a great fountainpen ink, a beautiful black but its NOT waterproof.
I also tried the one from Higgins, and its more like a dark grey and I'm having a hard time drawing with it. It has a really bad flow.

[CCThrom]

Generally I stay away from fountain pen inks... they're all a little dodgy in my book, and I don't have much good to say about Higgin's ink at all. I do however have a couple of thoughts... have you tried a tech pen ink? Some of them are nicely waterproof and if they can work in a technical pen I'm sure they can work in a fountain pen. Barring that, try FW acrylic india. I don't know if it will specifically work in a fountain pen, but it has good waterproofness and it's much thinner/more watery than regular india ink. (If you're up for the experiment, don't use your favorite pen though... might not work)