As you have suggested, find the right carbon seatpost for you - ideally one with minimal graphics and the corresponding carbon weave pattern to your other black components then sand the whole post to get that 'stealth' matt black look. Take it to an auto body repair shop for a quick and easy solution for them to spray it with clear coat again if you would prefer it back to gloss once you have removed the graphics.
I have used similar processes (both sanding and using a blade) in my 'Stealth' thread:viewtopic.php?f=3&t=110609
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That's pretty much it. I had the graphics removed from an FSA K Force Light seat post and then painted gloss. Matte is hard to get right. Did the same for the stem and the bars.
Local guy did it all for 50 bucks. Guys in auto body repair shops are usually up for making some easy money on the side.