if you're still looking for a black ink for your "51", here are my thoughts.
I live in the UK.
My experience of (UK?) black Quink is varied.
1) its feel on the page is great - the nib seems to glide along on a wet pool of ink, but the written ink dries on the page very rapidly, and;
2) I have always found cleaning Quink out of my pens to be very easy, requiring only a couple of flushes with cold water.
That said, I have not ever used it in my own aerometric "51", because of two of its properties that I find to be less-desirable:
1) I have found that in other pens (UK-made Parkers with UK 'medium' nibs) the ink tends to appear to be a very dark grey-green colour rather than a dark black, and;
2) the property that makes it easy to clean out of pens also makes it somewhat vulnerable to water/spilled drinks (I am a klutz).
I have run Pelikan 4001 'Brilliant Black' through my aerometric "51", and found that to be darker than black Quink and only slightly harder to clean out afterwards. It has more water resistance than the Quink, but it isn't an 'archival' ink.
Nowadays the only black I run through my "51" is Noodler's Black.
I love the ink's 'bulletproof' properties, and the fact that it is very dark on the page, with no evidence of 'shading'.
It doesn't appear to be 'shiny' on the page, but it is unambiguously black, and it is light-fast and spill-proof.
I have never had any clean-up problems from it
If you are in NY you ought to be able to find Noodler's ink easily.
My own "51" is a wet-writer with a UK 'fine' nib. Some people have found 4001 inks a bit on the dry side (Pelikan's pens write wet, so its inks tend to the 'dry' side). If your "51" writes dry or has a very narrow fine nib this may be an issue.
I usually change ink colours after each fill, and 'rotate' my pens after each fill, so my clean-up regime tends rather more to the 'OCD' side than most other people's.