Here is a BLACK case that I metered at "0" to me this seems more GRAY than BLACK and the reason for that is because it is overexposed.
A proper metering for a BLACK case or anything BLACK and DARK toned would be almost "-2" I say almost because you want to leave a little room for editing.
Here is a WHITE paper metered at "0" again we see a GRAY color and the reason for this is because it is severely underexposed.
A proper metering for a WHITE paper and anything WHITE or LIGHT toned would be almost "+2" again I say almost for a little wiggle room in editing
Here we have a shot including all 3 tones. Here I metered my GRAY card to "0" and this is the rare case where metering exactly at "0" is correct.
Here is a GRAY card metered to "0" and this is what it is supposed to look like. Using a GRAY card for metering is VERY effective and makes the guess work easier, BUT I suggest learning to both use it and not use it since not every shot is one where you can freeze while you run inside to get the GRAY card