James,
They are made of pigskin, and I have heard that they machine a little weird. Where a normal leather tip tends to "Curl" as you cut it (I mean the leather you trim off the tip curls), a K Black tip tends to almost turn to dust. This is just what I've heard, but I've heard it from a few sources.
The tip does have a different feel to it for sure. I don't know if it's because I like firmer tips or what, but the Kamui Black Medium is THE TIP for me. Nothing plays better in my book.
Noticed the same with K B, looks like it turns more into dust, sorta.
Might just be because of its black and proberly the treatment they do to get it black. lol