The Ultimate Blue, Bulletproof Ink.
581465
Noodlers Luxury Blue is probably the best blue ink ever made.  It's highly bulletproof, glows under UV light, waterproof, impossible to
remove from paper or cloth once it drys, even with the strongest bleach, acetone or anything else.  The color is very nice, just about impossible for a printer to duplicate.  By any standard you measure or test an ink by, this one comes out on top.  I _strongly_ recommend this ink.  It's expensive as all Hell, but well worth it.  Keep it and the pen you use it with in a safe place for those important official documents, checks, wills, deeds and any signing that's important to you!  I suggest an Jinhao pen or a good pen with a flexible nib or a music nib.  (Note:  If you need a bulletproof _black_ ink, Noodlers Bad Black Moccasin is recommended.)
2879885
I have used fountain pens since about 1990. I was a devotee of Levengers "Cobalt Blue". Its color stands out and you know its an original signature with that vibrant blue color but.... a single drop of water destroys it. This is common to nearly all fountain pen ink. If you address a letter with regular fountain pen ink and the mail carrier gets caught in the rain, they won't be able to read the address...if you sign a check with it - you better pray that a bad guy doesn't get a hold of your check as they can wash the ink right off...

Noodlers black waterproof ink is truly "bullet proof". You can sign checks, address envelopes or write your magnum opus with it and not fear your lifes work being destroyed by a rain shower.

When you open the bottle for the first time - be careful!! Noodlers wants to give you a full bottle and that's just what they do!
I highly recommend the stuff.
993234
Aurora black is my favorite black ink above any and all others.  It's deep black that dries deep black, excellent lubrication and all my pen/nib combos love it.
1561536
Wow! This is one of those blues that must be seen in person to believe, a very vibrant blue I have found that it does bleed on some cheaper paper but it's something I don't mind because the blue is so bright, This is my everyday work pen because of the deep blue color. Word to the wise this ink does stain so have one pen just for this ink, I am using it in a converted eyedropper platinum preppy. If you like a vibrant blue get this one!
764161
I love the blue ink that flows from these superior Uni-Ball pens made in Japan. I love the way they write and the way they fit in my hand. The black ink ones have been my go-to pen for a couple of years now, but, I am going to be adding the blue to my permanent ordering rotation.
844090
A staple black ink, this ink will hold up to the toughest of chemicals. Extremely saturated and pretty feather resistant, this is a great all purpose ink, useful for students to businesses and anything in between.
1072957
There are a lot of inks out there that have a few redeeming qualities that really outshine any drawbacks there might be.  Not so with the iroshizuku inks, I really don't have anything bad to say about this ink.  It is a nice bright rich blue that is dark enough to write with daily but not so dark that you would ever mistake it for black. There is a "lighter" shade that is really just a more muted blue called tsuyu-kusa, and a "darker" one called tsuki-yo that is darker but still muted. for me this is a happy medium with a little more saturation.

It writes really well in a number of pens on most paper types.  Wetter Pens really give depth to the shading that this ink is capable of. It really brings out the finer (and not so fine) qualities of the paper that you are writing with because it is a very liquid ink, It dries well and yet doesn't easily dry out your pen. definitely my favorite blue.
4301583
The ink flows ever so smoothly onto the paper and dries where it lies.  It doesn't blot, run or smudge, unless you spill something on it.
502554
I decided to buy ink vs cartridges, what a BIG DIFFERENCE! I lave "Heart of Darkness" not only is the ink smooth but it bulletproof. Its nice to see a "Large Bottle" and a nice pen.
2256448
I have an Epson R3000 printer that uses UltraChrome inks. I have been doing photography for 70 years. I find UltraChrome inks to be superior to even Dye Transfer dyes.
3253301
This is a great blue ink.  It flows / cleans up well and has a nice bit of shading.
Do know that the bottle has a unique inner-cone design where you can `the last drop` out, but it may not be deep enough to fill larger pens (#6+ size nibs).  For those, I'd recommend removing the inner cone before filling.
4116861
This is a magnificent ink. It hits the page a gorgeous blue, but as it is an iron gall ink, it dries into a deep, rich black with hints of blue. I use this primarily with dip pens for my calligraphy, but I also use it in my fountain pens that are easily taken apart for cleaning (Noodler's, TWSBI, etc.). I can't recommend this ink highly enough!
2489507
This is some of the best ink you can buy. Well behaved in every pen I have filled with it, shades and layers beautifully. The wetter the pen the richer these colors get.
340847
Great product. Low dust, low jamming, great ink retention.
1699309
I'm a beginner with fountain pens, but this ink has already become my go-to ink. The lubricating qualities of the Eel ink are excellent for fine and ultrafine nibs, the color is a good solid black, and it actually dries quicker than most gel pens I've used. Note: be careful when opening the bottle the first time, as it is filled to the rim -- the bottle itself is clear glass.
3188762
good ink,  capabilities for the high yield are true, wouldn't use anything else
3908311
I have to say I love this ink. It is super black! I have tried other brands and they have ended up being a greyish-black. This is true black. I have not had any problems with clogging in the pen at all. I love that it is waterproof on paper but cleans up easily with soap and water on other surfaces. I spilled some on my hands when I first opened the bottle. I thought I would spend the next few days with black all over my hands but was pleasantly surprised when it washed right off. I have found it to flow smoothly with my pen. I can definitely say I will be buying this again when the time comes.