Diamine produces fountain pen inks that have really nice colours. I bought a Diamine Golden brown ink for my vintage fountain pen some time back and I still love it. The problem with most fountain pen inks is that they are not waterproof. So I was on a hunt to find a good waterproof ink that I could safely put into my fountain pen. I found the Diamine Registrar's Archival Ink online and when I heard it mentioned that it was safe for fountain pens, I got it. The ink is said to be iron-gall ink and it is used by registrars and clergy for signing official documents because the ink does not fade over time. I was going to use it for drawing. So as long as it was black, waterproof, and won't kill my fountain pen, that was fine.
In my review, I used my Noodler's Ahab fountain pen since it's easy to clean if it ever does clog.
This ink had an interesting quality. It appeared blue in the bottle. And even when I put it down on paper it's still blue but turns toward black after a few seconds. The black isn't a solid black. But rather a dark grey. How dark it turns out depends on how much ink your pen dispenses. More ink in your line creates a darker line. Personally, I wasn't impressed with the darkness of the black. It wasn't a very attractive dark grey to me either.
Now for the waterproof test. I've read that iron-gall inks take longer to dry so I gave it lots of time. As you can see, even after 5 minutes, the ink still bled out a little when mixed with the water.
Next, I did a drawing. I made sure the lines had lots of ink so it was black enough. Then I left it to dry. After 5 minutes, I ran a paint brush with clear water over it and it still bled. Because I laid the ink thickly this time, it bled more this time. At least the lines did not disappear. It just bleeds.
Personally, I would not recommend this ink for drawing with watercolours. Definitely not. It bleeds too much.
In conclusion, I'd say this ink is only partially waterproof. It is not very black. The blue to black transformation is interesting. But I feel it does not contribute to the drawing experience in anyway. Perhaps it is still best used for signing official documents.
The ink is available on Amazon here: https://www.amazon.com/Diamine-Registrars-Blue-Black-Bottled/dp/B003OXNB...
Search Jackson's Art Supplies (UK) also.