A while back, I saw that Edison was offering double-ended fountain pens. This seemed too handy to pass up. I carry an unfortunate number of pens with me in my pocket and in cases, all filled with different inks (what's the point of having a fountain pen if not to explore the vast array of inks available?) and this would be the classic two-fer. So, I ordered it in blue swirl ebonite and waited patiently. The clip end is a medium oblique nib, and the other end is a regular fine nib. It is a dropper filler.
This picture does not do it justice. It is quite dark, perhaps to be taken for black. but the color changes under different lighting. It is darkest under incandescent, and it is most blue in daylight. This in itself makes it fascinating to look at, beyond the usual hypnotic swirls of color in ebonite. It is a bit long for the shirt pocket, but does fit in some suit jackets, and it is comfortable in my large hands un-posted. As is usual with Edison nibs, they are adjusted perfectly for a good wet flow. I was concerned that if kept this in a pocket in a vertical orientation, it might drool in the end with the nib down, but this has never happened. I was also concerned the flow might be a problem when left without writing for a while. This has also never happened. It writes immediately.
I did experiment with inks for a while, and had an unfortunate need to disassemble the whole clip end to clean out an ink that I thought was well behaved. So in the interests of that never happening again, the oblique end has Sheaffer Blue, surely one of the most benign inks around. And I really genuinely like the color, having gone through quite a number of blues over the years. In the other end, I have Noodler's Antietam rather than another red. Sheaffer's red is a true red, and I thought about using it, but opted for the dried-bloody, rusty Antietam. It has no special properties, like never freezing on the surface of the moon or being so perpetual the writing remains as a wireframe even when the paper dissolves into dust. It is just a very nice ink in a unique color. 
So these two inks with these nibs cover the majority of my writing needs. So why do I have so many pens? I am not quite sure now.

In my never-ending search for the perfect blue ink, I have found that the ones I prefer fall into families, or sub-categories of blue. That means it will be impossible to find the 'perfect' blue, unless I favor a family above all others, and I don't. However, I can find favorites among the families, and one family would definitely be cerulean.

Defined by Wikipedia: "Cerulean, also spelled caerulean, is a color term that may be applied to certain colors with the hue ranging roughly between blue and cyan, overlapping with both. It also largely overlaps with azure and sky blue, although cerulean is dimmer." The inks so classed are generally on the darker side and not like the sky colors, unless toward the end of day but before the pinks and purples of sunset. I am no expert. They exhibit a certain blue brilliance rather than the darker aspect of the blues, graying waters or storms or gathering darkness.

The two that I find to be exceptional are Noodler's Midway Blue and Pilot Iroshizuku Kon-peki. The Midway has a bit of green to it, perhaps less deep waters, while the Iroshizuku is definitely a deep sea blue. They both share a vibrancy of the sea, and I have had people comment on the attractive color of both inks. They behave well, and I will leave it to others to do proper reviews.

The examples above were on Springhill Vellom Bristol-cover Plus 16000 custom cut 3 X 5 cards written with an Esterbrook Jackson Stub #442 in a Koh-I-Noor #127 dip pen in my usual bad handwriting.