What was that thing that looked like Ghost in Robin Damashii, but was all black except for the Robin Hood parts?

Robin Hood's spirit.

That happens whenever Takeru goes into one of the Heroic spirits eyecon, in this case Robin Hood. The same thing happend in this weeks episode (ep 15) Were in (Spoilers)......... Takeru goes into Musashi's eyecon and it looks the same.

Anonymous it was the soul of Robin hood

It has to do with the current plot of connecting the souls of the heroes. So each soul form has a base form to interact with. It's kinda a nice detail instead of just having CGI ghosts talking with Takeru, you get a bit more of a real interaction.

it's robin hood

Real Robin Hood's ghost, I guess.

That's Robin Hood's soul giving Takeru his challenge.

It is robin hood soul itself,takeru will meet robin hood just like when he met musashi in the eyecon

They're literally just rushing out forms so much and then there's going to be nothing left to show off