Wouldn't anti-black coat be like some how realted to Sora. In DDD he came out of Sora from that seen where Riku was trying to wake Sora up. Soooo is like part of his heart or is he the darkness sora had in him. But didn't Sora's heartless have darkness in him too. So Sora must have consume more darkness. But how?????

There is darkness with in every heart.

Even sora has some. 

I believe it's just the embodiment of his nightmares.

 Riku fights a Nightmare wearing a black coat in this world, just who was that?

Nomura: That Nightmare was the personification of the nightmare Sora was having.

 

Sora was deliberately tricked into the depths of darkness in sleep so he could never wake up so they could use his body as a vessel .This figure was the personification of the nightmare Sora was having when this happened 

No. Anti-Black Coat is Sora's nightmare personified. Anti-Black Coat Nightmare was creating Sora's nightmare and keeping him trapped in it. It fought Riku as a last defense to prevent him from waking Sora up. If anything, he's made to mirror Riku more than Sora, with his body type being the same as Riku, his battle stance being similar to Riku, and the fact that it's Nightmare emblem is located on it's back just like Riku! But that doesn't mean it came from Riku either. It came from neither Sora nor Riku, it was planted there by the Organization. Although the form of the Nightmare probably DID come from Sora's dream itself, as he did encounter A LOT of people in black coats in that dream, plus, again, it was a trap from the Organization, so it's fitting that it would take on the guise of an Organization member, or better yet, an Organization impostor, yet another quick insult to Riku. Either way, though this entity draws a lot from both Sora and Riku, it isn't born from either of their darknesses. But it is a bad-ass enemy nonetheless.

It was the nightmare Sora was having .

Sora has darkness. People just keep denying because they don't want their favorite hero to have a not-so-good side to him.