I've always thought that the Tamiya NATO black was a dark green, not blue. Take a drop of the paint and add a couple of drops of thinner and you'll see what I mean. It kinda reminds me of "black" easter egg dye; no matter how long I soaked those eggs in "black" dye, it would come out dark green.
I mix my own scale black with Tamiya flat black and dark gray (about 50/50). However, NATO black does look good on tires for dark green or olive drab vehicles. The green tint of the tire ties in with the green of the vehicle. Of course, that's just my opinion.