Rick - I saw a photo the other day at an A/C website - can't remember the place I was at. They talked about something called "black death". It's supposedly more prone in Ford units - teflon was used in the compressor. Here's some text info I found. I realize your Benz does not likely have teflon in it's A/C system, but your may have a similar situation.

http://www.autoacrepairs.com/index3.html