The Attic

Whoa, what was that weird security check screen I got when posting?

It's a coke-ism I'm afraid. The line you're referring to - I believe, since it's the only one I can find in Coke where he describes Shiki's jacket as black - is in Overlooking View. Coke writes:

Originally Posted by Overlooking View

I took a black leather jacket with me when I left the house, and now I wear it atop my light blue kimono.

The original line is:

Originally Posted by Fuukan Fuukei, 1/Ryougi Shiki

家を出る時、浅葱色の着物の上に赤い革製のジャンパーを羽織った。

Emphasis added. The jacket is clearly described as 'red' by Nasu. Coke just fucked it up.

I don't know about Boogiepop. That seems more likely to be a different issue, where the colour is actually black but the artist depicts it as a dark shade of colour for stylistic regions. See, notoriously, Ciel's hair in Tsukihime.

Maybe I'm a bit late for this but I'm pretty sure the early editions of KnK have her jacket black, I once compared a few parts of the edition included in the cd of Tsukihime Plus Disk with the 2007 edition and the latter changed the description of the characters to match the artwork. I believe the 2004 edition didn't change the text at all and Coke used that edition.