As jOhO mentioned, there is some ambiguity in the way the question was phrased.
I am really not sure if I got the question correct. Was he trying to photograph a black cat? Or was it that the cat (whatever "degree of lightness" it was) turned out black? I assume the more likely one is the former, because he was trying to shoot some "black object".
Looking at the posts/replies, I think sweat100 got it most correctly, although his advice to underexpose by EV-1 to -2, (or overexpose by +1 and +2 for snow) is being too cautious. Eventually will work, but -1 is not too correct.
If one wants to take a picture of a black cat so that the cat turns out properly black, he should underexpose by 2 stops. I am afraid that Stoned got it completely wrong. And if espn wants to photograph white snow, then he should overexpose by about 2 stops. While Spectrum's approach will eventually get the correct picture, it shows a lack of understanding how light and meter work together.
sweat 100, shinken, and canew had explained the gist of the exposure. And Isisaxon's advice of using an incident meter should also work fine.
To explain why one should underexpose by 2 stops for a really black cat, and overexpose by 2 stops for really white snow, I have to revert once again to the zone system.
When one meters a black cat, the meter detects the cat to be too dark. The meter is calibrated to make everything grey. So it increases the exposure to make the cat grey. And when one meters for the snow, the meter detects the scene to be too white, and reduces exposure to make the snow grey.
Why 2 stops +/-?
The zone system gives a very simple guide to understanding tones. At the middle (which all meters are calibrated to) is 18% grey/middle grey. This is zone 5.
Zone 4 is more grey. And zone 3 black but with discernible texture. Zone 2 completely black with no texture. Zones 1 and 0 very, very black.
Zone 6 is lighter than 5. And zone 7 white, but with discernible texture. Zone 8 white with no texture.
Using the black cat, if one underexpose by 1/3 stop, one is hardly doing anything. Underexposing by 1 stop will make the cat a dull neither here or there "black". Underexposed by 3 stops will make the cat completely black with no texture. So the "correct" exposure for a black cat should be minus 2 stops.
The same principle will apply to photographing snow, but in the reverse direction.