What's the black thing at the bottom of navel oranges? Sometimes I'll cut into a navel orange and notice something black inside which originates at the bottom of the orange, where the navel is. What is it?

I have seen it get pretty far into the fruit too. Like almost half way in... is it safe to eat an orange like this?
I'm not as familiar with citrus, but tomatoes and peppers can develop something called blossom-end rot. This is caused by the plant not receiving enough nutrients (specifically calcium), whether b/c the plant isn't getting enough water and thus can't get the nutrients out of the soil or b/c the soil is deficient. 

Regarding whether it's safe to eat, a tomato w/ blossom end rot is still safe to eat, once you remove the affected area. 

I hope this is helpful! 

Do you have a picture of the orange in question? That would help greatly with diagnosis.