Delayed 2nd response because I had to fill a pen; I would say Chou Kuro is about 10% "blacker", mostly due to being much more matte than the Carbon Black and due to hiding the paper better. They're still both extremely black inks that behave very well in fountain pens.

Some #tengwar using a 1.5mm stub nib and #platinum Chou Kuro ink. Easily the blackest and most opaque black ink I have seen that's also fountain pen friendly. #fountainpens #calligraphy