I can't find the RGB and CMYK values for Pantone Neutral Black U on the Pantone website.
rgb.to says it's CMYK
0, 0, 0, 100 but I am not sure if that is right.
Anyone know what CMYK Pantone Neutral Black U has?
Keep in mind that any CMYK or RGB values given for Pantone inks are essentially made up. Pantone do spend a lot of time and resources on making their colors and conversions as accurate as possible, but the point is there are no correct values. Even the values that Pantone give change over time. For a start RGB and CMYK values in and of themselves aren't colors; they can change depending on the color profile and will look different depending on the output. Pantone inks on the other hand are designed to always be the same, it is a color matching system after all.
You're right that 100K is pretty far off Pantone Neutral Black U though.
We can get a more authoritative conversion by looking at the Pantone color libraries in Illustrator (or Photoshop or whatever). As far as I know, the Pantone libraries only actually contain Lab color values so the conversion from Lab to CMYK or RGB is done by Illustrator itself; but that is a good thing since it will give you values based on your color management settings, rather than blindly giving you set values (try assigning a different color profile and you'll see it gives you different values).
Anyway, this is what I get based on my current color management settings (which probably aren't the same as yours):
C:60 M:53 Y:53 K:52
R:76 G:72 :B69
Don't blindly rely on someone else's conversion (even Pantone's) though, a lot of Pantone inks simply can't be reproduced faithfully in CMYK or RGB. Do your own testing and use what works best (I can't do a test print now but I've got a suspicion those Neutral Black CMYK values will look a lot muddier when printed).
I got the answer from Pantone's TechnicalSupport (very quick support):
The three Pantone colors you have listed are base inks from the Pantone Goe system which is discontinued. Six of the 10 base inks used in the Pantone Goe system were brought over to the Pantone Plus Series but they were renamed. Below are the new names and I would recommend using these colors and you will be able to find the information you are looking for.
I have found that rgb.to doesn't really give me the best results when trying to convert a pantone colour. I usually use https://www.pantone.com/color-finder or the Pantone Bridge library.
On the Pantone site the values for Pantone Process Black U are:
RGB 78 74 71
CMYK 0 0 0 100
I believe the Neutral Black swatch was removed from the Pantone swatch guides in the last couple of versions, but I could be mistaken. I have an old swatch guide with it, and a new one without the swatch.