Something I noticed about Black Chidori

[Behemoth55]

Do you think maybe its a sealing technique or what?

No, it's not a fuin in this context, but Sasuke changed Chidoris form a little bit due to Keitaihenka so he could restrict Madaras movements. That was the basic idea. Sasuke even said that.
The basic characteristics of every jutsu(which base on a Seishitsuhenka) is everytime the same,-like in this case it's only Raiton-chakra anyway-however, due to Keitaihenka those jutsu change its form.
Basically, Chidori is used to breake through any kind of defense, but as we can see at your scan the Raiton-chakra looks like a cage, right? As I said before, that's Keitaihenka.

 

[IceBreaker]

It's simple,Sasuke now has better Raiton control then Kakashi at that time.