Iamchappy posted this link in different but related thread:
http://www.pelicanparts.com/techarticles/rice_ramblings/rice_ramblings-4.htm
It's worth reading the whole thing, but in the first paragraph it says they ARE black and sooty.
Now, my 930 is stock with the W3DP0 plugs. I just removed them and they were ALL black and sooty. But this is normal.
What plugs are you running and what is the gap? Maybe this plug isn't gapped correctly. Dumb question, but are all the plugs and the gaps the same, because you said you swapped it out?
Possible fuel injector issue?