I have no experience of Online Nachtblau.
But, if you desire a greyish blue-black, Rohrer & Klingner ‘Salix’ starts out on the bluer end of the blue-black spectrum, but it ‘cures’ to a grey-tinged colour.
If you find that ink to be too ‘blue’, I would also recommend Pelikan 4001 Blue-Black, which also turns grey-ish.
Or, for a darker blue/black, Pelikan Edelstein Tanzanite (although that ink isn’t really very ‘grey’, and is not ‘inexpensive’).
After you have received a few recommendations for suitable inks, I advise you to have a look through the ‘Ink Reviews’ board.
Its ‘index’ thread is a good place to start . Linky!
Another good place to start looking for suitable inks yourself is the following thread:
N.b. I advise you to read/watch several reviews of any inks that get recommended to you.
That is because:
different people’s cameras/scanners render colours differently
different reviewers use broader or narrower nibs, or ‘wetter’ or ‘drier’ pens
different reviewers use different types of paper
Ideally, you want to see reviews by people who use the same types of pen that you do, the same width/grind of nib that you do, and who write on the same type of paper that you do.
But of course it is highly unlikely that many people will use all the same ‘equipment’ that you do, so reading several reviews is a good way to get an idea of the ink’s range of behaviours from different pens/nibs/papers etc.
That said, I know that @namrehsnoom uses Lamy Safaris in different widths to write most of his ink reviews, so his reviews will be a good place for you to start. Especially his review of Pelikan Edelstein Tanzanite 🙂
I also recommend the reviews by @Sandy1, who was for many years the most-thorough reviewer of inks on FPN, and who is much-missed.
Of the inks that I mentioned before, she reviewed e.g. Pelikan 4001 Blue-black and R&K Salix.
Another reason to consult several reviews is because different reviewers notice different things:
is the ink ‘wet’-writing or ‘dry’-writing?
does the ink shade/sheen?
does the ink feather/bleed?
is the ink difficult to clean, or does it clog pens?
does the ink cause nib-creep?
Reading/watching several reviews of any ink gives you the best chance of being alerted to anything that would be a ‘deal-breaker’ for you before you buy the ink.
If, after you have read several reviews and want to try out an ink before buying an entire bottle, many retailers will sell you a ‘sample’ of a couple of ml of the ink.
I wish you good luck - and good fun too - in your search
Slàinte,
M.