i have five choices thus far recommended: Platinum Carbon Black; Pelikan Fount India; Sailor Nano Black; Pilot Iroshizuko Take-Sumi; Noodler's Manhattan Black.
using online comparisons appears to me to be Sisyphean labor. so then what kind of answers am i seeking? - i can only answer that i'll know it when i see it.
It's important that it not harm my vintage Parker 51 ... and herein lies one of the rubs because some members posit that a named ink harms and others ragefully refute the claim.
Does FPN have a beginner's glossary. bulletproof; wet/dry nib; ... ?
First, very little will harm your Parker 51, other than possibly stain the collector (which you can't see anyway). The only hassle is cleaning out the pen to change inks. But that is a function of the truly brilliant design of the collector. It has far more, and far finer, fins than any other type of pen.
Second, the Carbon Blacks (Platinum, Pelikan, Sailor) are truly black (in a quantum sense) but don't look as black as some other inks like Noodler's Heart of Darkness.
It also depends on your paper. The Carbon inks will work better on poorer papers as they don't tend to feather or bleed. If you were going to use any of the above, I would recommend the Sailor, but it is more expensive than the others. I found the Pelikan Fount India a bit more troublesome than the others, tending to dry out in the nib a bit more.
There are many, many, many blacks. Aurora Black regularly gets a mention as the non-permanent black ink to die for.
So, of the Permanent Blacks, I would recommend Noodler's Heart of Darkness (it is a thicker, richer, more lustrous black than standard Noodler's Black, but will bleed more) and Sailor Kiwaguro Nano Carbon black (perfectly behaved ink, but not as glossy black).
If your pen is reasonably wet, then standard Noodler's Black will be fine. My Parker 51 is a bit dry, so desires wetter inks like Heart of Darkness for Black or Private Reserve American Blue for Blue.
Of the Non Permanent Blacks, I would recommend Aurora Black (simply on other people's say so, as I haven't tried it) or Diamine Onyx Black, which I have tried.
Others will have their own recommendations. Look through the Ink Reviews section, balance up all the competing/conflicting opinions and then finally buy the ink that appeals most. Then another, and another, etc., etc., etc.