Not sure how or why it would smell of sulfur. I've heard the special ATF is much like Dexron, though the additive package differs.

The black is normal. The clutches in the transmission include graphite, and the fluid picks it up over time as the clutches wear. Inky black is not a problem, though it does indicate the trans has not been serviced for many miles.

The filled-for-life thing was an MBUSA policy. The Rest-of-the-World always had a 40K-ish mile service interval for this transmission.