I was cutting up a mango, when I noticed some black threads inside.
What are those?
They are quite tough, brown-black, about an inch long (2.5 cm). I've got a picture.
It's an indication that the fruit has started to rot:
Edit: Yes, the above is probably being overcautious in stating “any black fibers? → trash!” It's from a “Dummies” book after all. ;) I couldn't really find another good reference. But I'm sure you can make the distinction between a piece of fruit that has a small brown spot and one that's black and shriveled. Still, the color of the fibers is a result of the late ripening/rotting process of the mango, rather than a result of an earlier stage in its growth. But having eaten this mango with the one, tiny black fiber shown in her picture, I don't really expect to read Mien's obituary on this site tomorrow. ;)
I believe some mangos do that in early stages of over-ripening (similar to how an avocado does that). I usually cut around them and keep eating.