Which type of jet leaves black trail at cruising altitude?
That could easily be a military jet, or it could be an older type passenger jet. The old jet engines had inefficient fuel injectors and simply used a lot of them, creating a higher fuel-to-air ratio which results in black smoke from carbonised fuel. There is only a short time to mix the fuel with the air under pressure in the combustion chamber, and modern day engines pre-mix the fuel with air before injecting.

Olden days jet engines also used water injection in times of high demanded thrust, creating more black smoke due to a cooler combustion chamber, but since the aircraft you saw was in cruise it would not be in a water injection flight phase. 

So any older type of jet engine could have caused the black smoke you saw. Could have been an old DC-9 or even a B-52 :)