Black layer in an onion I just sliced an onion and found that there is a large section of one of the layers that's black. What is the black substance? Is it safe to eat? If not, is the rest of the onion safe to eat?

The onion in question:
This is either mold or bacteria-caused rot. Don't eat it - the microorganisms which feed on raw plants are rarely pathogenic for humans, but will frequently create toxic metabolic byproducts. I would throw out the whole onion, molds can be toxic in very low doses, just from contamination between layers.