Platinum Black Carbon Ink
this is by far the best and darkest fountain-pen ink i've found that is also waterproof. it lays down a matte black line, hardly any shading that i can see, and can stand up to layers of watercolor once it's dry on the paper (and it doesn't take that long, depending on the paper). i usually get them here at jetpens, but can also be found on ebay and various online sources. (i'm not endorsed by jetpens or anything, but i like their service, and they are excellent, responsive vendors.)