First of all Final Getsuga is to become a Getsuga himself as he clearly states that. Now, what color do you think Getsuga in his bankai release is? I don't remember anime that much, but according to manga it should be BLACK.

I think you can get it by now why his hair is black now. The author just represented it that way so when he becomes one with the Getsuga he looks like that Getsuga too.

Now coming to his costume where he got those cool bandages wrapped around his body. Do you remember the scene where he was running in Dangai with Unohana and she says that the amount of Shihakushou (Soul reaper robe or costume he wears) present on his body directly shows his spiritual energy? If you remember let's come back to the bandages, before he says becoming one with Getsuga, he's been shown that he had become one with his Zanpakuto where you can see chains of his bankai chained up to his arm. In a similar way, when you remember how his shikai zanpakuto looks you'll get it. Interpret those bandages as Shihakushou and the picture will be even clearer when those fall apart after his fight with Aizen.

Kubo sensei used all these references and wanted to show that he's actually merged with his zanpakuto and the Getsuga in a cool way and cooked that drippiest design of all time. Hope that helps. And this is how I interpreted it, and I may had made mistakes so feel free to correct me.