I've come to like using Sheaffer Skrip blue ink, it's a brighter blue with quite a bit of purple in it (the above pic is a fairly good capture of the color).
Skrip is easy to find and comes in the standard array of colors (black, blue, red, green), but there is something about this blue that makes it stand out from the rest of my blues. It has a glow to it that I don't see in the J. Herbin Bleu Nuit, which is another "benchmark blue" that I use for color comparison. It also shades and covers well, with a lot more pigment than I expected.
The Skrip blue ink dries very fast, which is another plus. It isn't waterproof, but I've rarely needed that feature in any of my inks (never, actually). It's readily available, and looks good on paper (or fingers, if you slip while filling as I do). There's a lot to like in this ink, definitely give it a try.