I agree with CV880
Unlike internal combustion engines, black smoke in gas turbines does not necessarily indicate an inefciency in the engine cycle (fuelburn vs payload/cycle) it only indicates a variation in fuel/air ratio far below stoichrometric (in order to keep the tubine from melting)
The jet engine designers tinkered with all those little air holes in the combustion cans back in the 50-60's to get rid of the visible smoke for two reasons
1. It was easier to track fighter aircraft by eye whith those old type anti-aircraft
2. Folks on the ground around commercial airports associated black smoke with polution.
Bingo they got rid of the visible polution and now you don't know what you're breathing
