User blog comment:Theluckysakura/Kuro Kiku/@comment-1271618-20100831004046

Yeah, as others said, black uniform Japan is mainly used in fanart to depict him doing more bloody things. He's worn black in that strip with him and China, his first appearance in the webcomic (and some of the Japan Omake), and the volume 3 redrawn version of the driving strip from "Japan Omake". He's never worn the black in the anime though. The recent sketch Himaruya drew was the first actual time he did a color picture of him that way too!

Black Feli and Black Matthew are completely fanon inventions though. With the way some people write Prussia in fanon, there'd be a dark/evil Prussia too.