Have someone notice what is inside of black zetsu? I think its white zetsu only his outer part is black but his inner part was white. Maybe the black zetsu was overcooked that's why he became black? ^_^no just kiddin'
See the pic below the black and white zetsu are going to separate with each other, you can see that the middle portion was white like with white zetsu and you can see that black zetsu still have white...maybe we can assume that his inner part was white and this is from white zetsu......his skin is black but his inside part was white.....what do you think?=)