I’ve always been fascinated with fountain pens. I mean, it takes skill to write with one, so there is an air of seriousness and intent surrounding them. After all, technicalities aside, you don’t tweet with a fountain pen.
Also, the pen itself is an engineering marvel — an aestetically pleasing outer shell enclosing a carefully designed system to enhance a controlled leak of the contained ink via capillary action through the nib. I mean, really, when they wrote “the pen is mightier than the sword”, I don’t think they were referring to the cheap plastic pen attached to a chain at the post office.
The other main ingredient to the system is the ink. The ink is what leaves a mark on the paper — an expression of the writer’s intent, creativity, and expression. It is what conveys the words to the reader. And leaving writers aside for a moment, I’ll note that an octopus risks his life on the quality of his ink. So the choice of ink is not a trivial decision.
Though I’ll leave fountain pens for another discussion, regarding inks, I must say I’ve really been happy with the Noodler’s Heart of Darkness Black Fountain Pen Ink 
In case it isn’t obvious from the Joseph Conrad allusion, this ink is really black. Blackhole kind of black. It is also, well, bulletproof, in that it is meant to stick around on the paper for a long time. No worries about your snail mail address washing away when your letter drops into a puddle on route.
This package offered by Amazon
But the main attraction here is the ink. If you are confident in what you want to say, and want the security of knowing it won’t be lost or distorted, then choose Noodler’s Heart of Darkness Black.
Noodler’s Heart of Darkness Black Fountain Pen Ink