I have a 1998 EC 250 6 day as well and i also use 50:1 only when I go up in elevation do i notice the nasty black drip and that is because i am just running 1-2 main jets too rich. It still runs great but requires a little extra cleaning when I get home.
When was the last time you changed the muff packing? Once that is saturated you will see nothing but black splug out the back. If the bike has been running good with no changes to jetting or mixture oil and it gets worse each time out it's most likely your packing. Good Luck!