What is the black stuff that’s on or in breakfast cereals? Usually when I’m eating cold cereal, particularly chex, I’ll see at least a few with black or brown residue or specks, and set them aside to throw in the garbage since I don’t know what they are. Is it rat turds or something more innocuous?
Burned bits, iron, other minerals. This is GQ, and i just gave some WAGs. Sue me. 
Mouse turds are about the size and shape of a grain of rice. Rat turds are of similar shape, but much larger - roughly the total volume of about two peas.
Haven’t you ever put bread in a toaster? It usually pops out with a few black specks, and many more that accumulate at the bottom of the toaster.
Breakfast cereal is made in huge toasters.
Another possiblity: The Master Speaks, though the bits mentioned in that article seem to be smaller than what you’re seeing.
Rat turds, mice bones, pieces of cockroach, fingernails, you name it. But, then again, it could just be burnt bits. 
Yeah, it’s just burned bits. Don’t feel bad, I still pick it out and throw it away. I know it’s perfectly fine, but like you said it looks like something disgusting. I once had someone tell me she thought it was pieces of burned bugs!