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Black coming out of the exhaust is rich on gas.
Blue coming out of the exhaust is burning oil.
White coming out of the exhaust is water.
Grey or more clear exhaust is good.

I like to take my finger and rub the inside of the tailpipe. It should be little or grey in color. If it is black and sooty, then it is too rich. Of course it takes some amount of driving to get the tailpipe to a good color. That's why I would pull the plugs and look at their colors.

Start with some new plugs and make sure the timing and gaps (plugs and points) are correct, cap, plug wires and rotors are in good shape, valves are set right. After running it down the road for 10 miles or so, pull the plugs to check their colors. If you make a change to timing, carb, or anything, clean or change the plugs and drive it again and check the colors of the plugs again. If you think about it, the spark plugs are a part of the combustion chamber. That is the best place to look to see how well your engine is burning fuel.

I like to see the ceramic piece of the center electrode of the spark plug to be a nice brown-tan and grey. The ring of the bottom of the plug can be darker. Sometimes, debris on the plug that looks like molten or boiled crap on one side of the plug (this would take more than a few miles) would indicate a valve is loose in the guide and/or valve seal is bad, and allowing oil to burn. If you have access to a scope that you can look into the combustion chamber and the piston is washed off and clean looking around the edges near the rings, that would indicate oil is getting past your rings.

Let us know what you find.