I had the same problem with BOTH of my modern 100's (the vintage one is a workhorse!!) I sended one of the pens for repair and came clean and worked ok for a couple of weeks. I decided to go for it and fix it. Somewhere in the repair Q&A there is a thread with pictures of the cause and the cure I did (still working OK after months...
The problem is that the converter gets a bit loose and starts leaking ink. This ink finds its way up and with fillings it gets into the upper converter compartment. When acting on the knob, and every time more, this ink is expelled round the knob and directly into your BLUE or BLACK or whichever colour your fingers look afterwards. In my case I sealed the converter in place with a removable solution. There is no structure modification, just a seal added and fixed in place.
Search for CS 100 piston leak and you should be able to find it. The pictures are self explanatory.
In your case, I would also reccomend to claim within your warrantee as the kind of work I did on mines should end any warrantee (but for me... it works, as I'm far away and with the "repeated" postage money -to CS and back several times- I can get quite a few pens

)...
Now I'm quite happy with both 100's, they are marvellous writers!
Cheers
Jose