Start by replacing those spark plugs. It appears that the porcilin is dammaged which will cause misfires. They can't be that expensive. Next make sure that your points are properly set, this is best done with a dwell meter if you can, if not gap them at the proper setting. Then AFTER you set the points, set the timing, as the timing will change as the point gap changes. If one side of the points is pitted, and the other has a bump, change the condenser, a bad condenser can cause a misfire. Better yet, just replace them both, that way you know for sure, again, they can't be that expensive. Next look at the fuel filter and check for water in your gas, this would cause the apperent rich condition (black plugs) and a misfire at the same time. Check the float level, too low and it starves for fuel and misfires, too high it floods and misfires. Another thing to look at is the valve clearances, if they are too tight, it will cause the valves to remain slightly open and misfire, this is most common on exhaust valves. As you can see there are a lot of simple things that could cause a misfire.