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):   
It's an indication that the fruit has started to rot.  When dark spots start appearing on the side of a mango, it's starting to rot. A mango can rot from the pit, the skin, or from the non-stem end. Any black fibers also indicate that the fruit has started to rot, and at that point, you need to toss it into the trash. From Student's Vegetarian Cookbook For Dummies, page 301  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. 