We managed to sail her all the way home because I was afraid to try to start her without some indication of what the problem might be. Wouldn't want to make things worse.

When we first noticed the soot, 2 days earlier, I replaced the alternator belt thinking that might be the source of the soot. After it shut down there was soot in the same places I had cleaned.

I hadn't thought of the air filter, but that's a possibility. Since writing this I researched some articles I have and the thought occurred to me it could be a clogged exhaust elbow. The whole exhaust system is about 20 years old.

The engine shut down was very quiet; no bang or bad noise. My wife says she thought she noticed it had lowered the RPMs a bit just before it quit and she had just checked the instruments - normal.

My wife agrees with you; check all the easy things first.