Many of the paints you have mixed at local stores comes out looking various shades of gray. A paint rep once told me why, but I don't recall.
The short answer is 'you can't get there from here'... Trying to mix a black from the primary colors in a tinting
mixer just doesn't work, you have to have true black
pigment, and lots of it. If you think of a color wheel, and picture white to black as an axle through it, when you only have certain colors available, you can only mix hues that are towards the center of the color wheel, or, to put it another way, green + orange does not yield yellow. To get any pure color, you need the pure
pigment, and with non-black pigments, you just can't mix them dark enough to become a true black. Sometimes a clerk will try, but even if the tinter is stocked with black, they won't believe how much they need to put in.
I've always just used whatever's cheapest. The platforms I have in
stock now sometimes get used outdoors, so I use exterior grades to protect the wood, but otherwise I don't spend more than I have to. When I was at a community theater, we'd leave the floor however it was painted for the previous show until we had a need to take it back to black - figured it was a waste of time and paint to do so if there weren't any events in between shows with scenically painted floors.