Need to be a bit careful here.
Fuel injected petrol engines, generally have a black exhaust, yet are significantly cleaner in terms of emissions than cars from the 60's. The readings that you may have encountered could be referring to old carburetor engines. Although dirtier, when tuned correctly they output a grey colour exhaust, if black, then they were running too rich. A good guide is to look at similar cars and engine types. I think you'll find that black is common. Remember that you are observing the coldest part of the exhaust pipe. You will find that there is not any soot up near the engine if you were to open the pipe. When the engine is running, you should NOT see any black or blue smoke.