A particularly harmful cigarette
. Unlike normal cigarettes, a black lunger is usually either unfiltered, or the cigarette is very harsh. Thus called because it's believed that, since they are worse for you than normal cigarettes, they blacken your lungs faster.
Person A: "Hey, I noticed you stopped smoking Marlboros."
Person B: "Yeah, I'm smokin' Lucky Strikes now. They're real black lungers."