99K isn't a GM paint code, that is the part number for the black toner from Dupont. Blacks on cars from the factory come from different companies, not all of them have a nice black black. Not only that the carbon was taken out of the black back in the late 70's and there hasn't been a REAL true black since then realistically. Be it the paint that they have to produce for the manufacturer to use in the methods they use, or they simply don't have a very black black in their toner available. So the "black" that is found on many cars isn't black at all as you point out. It is extremely dark red, blue or brown. And believe me, paint a door with a red black on a car that has a brown black and it is VERY noticeable.
The "blacks" you find on these cars have a formula from other paint companies to match them, they will literally have five or six toners just like some metallic green or something.
That 99K is right off the mixing bank, and to tell you the truth I have no idea how black it is being I am not using it or even painting anymore using different products so I don't know. I know that the last time it really mattered to me I went around to a couple of paint stores and I bought straight black toner from them and check them against one an other and 99K wasn't very black at all, S-W kicked it's butt. I ended up using S-W and was very happy with it. But this was years ago and may be totally different now. You are going to have to do some serious home work or trust someone a lot to go with one without going out and finding out for yourself what is the blackest black you can shoot your car.