I have mentioned this before but just in case you missed it...
The ECMs in my 2002 Flame where swapped out under warranty in 2003 for this very problem. If they don't acknowledge the problem then they are sweeping it under the rug. They may also tell you that your out of warranty which would be hard to argue I suppose.
Bottom line is that there is a problem and is fixable but may be a warranty clause. After the swap out the black soot has never returned.
Dave