you get yarn.
The black cat produces yam.
Yarn
yarn
Same way you get everything else in FarmVille. A friend has to fine a lonely black cat on their farm, post it on their wall, then you have to click on it before all of your friends do.
It's like a ball of yarn for the cat to play with.
brown and pink cow , clumsy reindeer , black cat , turkey , turtle , black sheep, and ugly duckling
black bacon :)
This is because the Farmville creation team created it like that.
Wool
If you have a cat, when it is 100%, you get the option to "brush kitten" (it's the kitten version of "harvest").
Harvest wool