L.N.
When you first mentions "black spots", I thought of bananas. I don't know if he's eating a lot of bananas lately and I"m not sure how small of spots you're talking about. I know if my son doesn't chew his banana pieces very well they come out as blackish areas in the poo and also the strings from the bananas come out and they are blackish in color.
If it is from bleeding from your son, it would mean it's higher in the digestive tract that the bleeding is occurring. Such as from the stomach or small intestines. If the bleeding was from the lower intestinal tract, then there would be red in his BMs.
These are some other things that can make it be black/greenish/even dark blue.
Black licorice
Blueberries
Iron supplements
Lead
Bismuth (Pepto-bismol)
If you cannot conclude that he has been eating any of these things (oh, blood also makes it "foul" smelling, which what poo isn't foul smelling?), then I would call the nurse on call for the Pediatrician (do you have a nurse on call). Or you could call the ER to talk to one of the nurses the to get her opinion. But definitely take him in tomorrow with a sample, but I would try to talk to a professional today if possible.