Bong pipe joints or what, and do you use a butane lighter. I find that the more times I light my stuff the more black spots I get in my sputum. Was ripping personal bowls in a wee bong that required a light every time and was getting ridiculously black and grey spit up. switched to a pipe and it was still bad, but a little less than the bong. Had the thought that part of it could be soot from the butane lighter I was using. (wave something in the flame and it will get covered in soot from the burnt butane) maybe your buddy chases the green with more flame than you.