First, a valve is not going to turn the fuel black.
Did you pump some fuel into a clear container to check? Or are you just confident in your filters?
Do you have any additives in the fuel?
The MMO just ran down an intake runner and is dripping out ~~ no worries there.
When she died did she hesitate and stumble or just quit like it was shut off?
Sooty plugs are indicative of a rich mix, poor combustion or both.
Dave Neptune