mark01 is right. It would probably be easier to fit a new seal. If you really wanted to clean the existing one, and you tried to clean it in situ, there would no doubt be odd corners where you couldn't quite get in and clean them. If the seal is easy to get off, I reckon that it would still be more work than it's worth to be scrubbing away at it, in the sink, with a bottle of bleach at your elbow. Save yourself a lot of work, and phone the Bosch people!
I asked the repairman about that the last time my Bosch was checked over, the seal has gone black in places. He advised me to put neat bleach on it then rinse it off.
Are you using non-bio detergent? This can really give problems with mould and smells - I thought I would need a new washing machine when I switched to non-bio, thinking my 20 year old machine was past its sell by date. Went back to bio detergent - problem solved!
I always use non bio as my OH is allergic to all but one type of washing powder and I've had my machine for 9 years doing at least 7 washes a week and have no problems with the rubber seal going black. It will clean with bleach, but you'd need to make sure you rinsed it off properly as it can corrode rubber. Might be worth putting through a decent hot wash empty to make sure it is clean and as said above, don't leave the door closed especially if you aren't using it daily.