I was recently travelling to Morocco, and as I was sitting by the window, at some point mid-flight I noticed another jet travelling in opposite direction at our altitude. The reason I noticed it easily, was that it was leaving very dark, solid, nearly close to black colour trail. It definitely was at cruising altitude, as after that we were still flying for about 1.5 hours before landing. I just can guess that it was small-ish jet, as height of its body appeared nearly the same as diameter of the trail. I couldn't see much details any way, as it was about a mile from us.
Does the black trail indicates fuel inefficiency of an engine, and that could be on of the old military jets?