I answered a post regarding this and went into a little more depth than I intended...
This comes up a lot so I am giving it a thread of its own so it can be found easier at a later time via a search.
Here's a little course on what makes a plug dark-black
When air and fuel burns completely and efficiently, the results are CO2 (Carbon Dioxide) and H2O (Water). Nothing more, nothing less.... neither of these has color to it. It's not much different that a fire log that has burned completely and efficiently.
If you have a fireplace or enjoy camping and have seen the results of a good burn, the results are just a fraction of what you started with...white ash left at the bottom of the pit.
I am sure you have also pulled black bits of log out of a pit just like you have pulled spark plugs from your engine that are black on the ignition end. The black is carbon. When carbon is produced so is Carbon Monoxide which can be lethal.
30 PPM Permissible Averaged over 8 hours
200 PPM Maximum for acute exposure
800 PPM Lethal < 2 hour exposure
Carbon Monoxide is caused by partially burning fuel. In its smallest form, a fuel molecule can be compared to a fire log. If you put 1 fire log in a pit and let it burn efficiently with enough oxygen to support combustion it would leave a white ash on the outside as it burns to the core. If it continued to burn efficiently it would leave nothing but a very small amount of white ash.
If you start to throw more and more logs (fuel molecules) in that fire pit you start to smother the efficiently burning log and the oxygen it was getting to burn efficiently is diminished. The fire will start to burn cooler and less efficient. The flame will lose its brightness and start to get more orange and the results will be the formation of black carbon.
A Carbon Monoxide Molecule is a partially burnt molecule of fuel. If you continue to smother your log (fuel) with more logs (fuel) the flame will go completely out and leave you with Carbon Monoxide and Carbon. We cannot see a Carbon Monoxide Molecule because they are too small, colorless and order less but we do see their residuals as the black outer carbon shell they leave.
Take your log out of the pit that was burning well but later smothered out and cut it in 2. What you will see is un-burnt log (fuel) in the center and a black outer shell (carbon). A partially burnt log is like a partially burnt fuel molecule.
The black on your plug is the outer carbon shell. If your mixture has never burnt well, you can wipe off the black carbon and find nothing but more black carbon. If your mixture burns well at high speeds but not at idle you can pull a plug after a long ride that has a short time to idle and find carbon that can be wiped off to reveal a white residue and a clean electrode.
Remember, a completely burnt log (fuel) leaves very little of what you started with... white ash (CO2 and H2O). A partially burnt log leaves most of the log with a thick black outer layer. This is why a good burning mixture leaves very little to nothing on the plugs electrode when a poorly burning mixture quickly starts to build up a black thick layer of carbon.
So put more simply, the black is carbon caused by the lack of oxygen to burn the fuel completely.
Although that may now seem simple, the cause(s) may not be. Causes can be:
Correct amount of fuel but incorrect amount of oxygen
Correct amount of oxygen but too much fuel
Lack of compression to support complete combustion
Poor spark to start a good combustion
Cold engine temps that don't support proper vaporization
Each of these can have a host of issues that cause them to occur that would require another page of explaining.