It could be because you're now driving extremely conservatively now.... as in not revving your engine hard or to high rpms in the weather conditions you're having there in Mass (to avoid sliding on the icy roads). That can load an engine with carbon.
I'd also stop wasting your $$$ by running Sea Foam through your engine every 3000 miles. It doesn't need it and you're not doing your engine any favors by doing that.
Now your engine might be running rich, if it is at all, from a bad upstream 02 sensor. It's the upstream 02 sensor that sets the air/fuel ratio.
Finally, it is normal for a muffler to drip moisture and that it could be sooty could just be some carbon from having to drive the engine so conservatively right now. You might try revving the engine hard to high rpms whenever possible over a few days, that might help to blow out any carbon that could be in the combustion chambers.