Those of us that study this will say the answer lies in 4 to 5 paragraphs, maybe 4 to 5 pages?
A plug check is beyond most of us here unless we set in a dyno room comparing notes and AFR meters.
I will always say "install one on the bike!
Aside from that-your plug reading will read what the engine was last doing. idling?
Most here have no clue how to do an actual plug CK, and I am not going to write it up.
I suggest you look it up?
I know you have a softail,,, what year is it, an evo or twin cam engine..
Dark and sooty, is almost always too much fuel, to little air, or both..
You should check the air filter,, and the carb, or efi system, depending on which you have.. the exhaust system, and ignition systems are also a potential contributor to the issue, but not usually the only cause..
More info = more assistance... diagnostics needs as much owner input as possible..
There is nothing snooty about my answer, you guys have feelings made out of glass ?
JC folks, all I said was, to write it all up about plug check would take an hour or so and it has been done so many times.
Go look it up!
I am not going to explain it all here about speed, load, , altitude, temps, clutch pull in etc etc etc
I did recommend he get an AFR unit, which is the best advice he will ever get.
Well, here ya go.
Would be nice to know what kind of bike. Engine generation. Whether fuel injection or carburetor. When was last tune up?
Sooty....Dry... rich fuel. check fuel ratio first. Check fuel mixture with vacuum gage if carburetor, or computer on fuel injectors
Sooty and oily, worn rings/pistons, and rich fuel mixture? Be careful of rich, which may lead to cylinder washdown of protective oil.
If carburetor, are you leaving choke on too long with rich fuel mixture.
Around town driving or open road?
Did you check plugs after riding around town, or after an open country road ride. I like to check plugs after coming back off a country road ride without quick town takeoffs.
If fuel injection, injectors tend to wear from lean mixture to rich mixture. Do you have 25k, 50k, 100k miles on injectors.
Any recent engine work done?
Soot doesn't mean as much these days as it did in the past. This fuel we have today leaves a black residue when rich that is very reluctant to burning off. So a lot of carbed bikes will show soot from the choke, even though they are jetted fine.
And since the OP has not come back to further expand on the issue, everything past the engine being fat is pure speculation.
Run your finger on the inside of your exhaust pipe (let it cool a bit) it should be somewhat sooty with carbureted models. If it feels oily check your crankcase ventilation. It's possible you have worn engine components.
You can go up a heat range on the plugs, ride the snot out of it and do the top end this winter