Black Coalish Thing In Orange
If the substance was soft and smeared easily, as you describe, it was most likely mould - sometimes cells die in a part of the fruit, often through exterior damage, and these rapidly start to decompose. Left long enough, the whole fruit would have gone black. Harmless so long as you don't eat the section that's got black in it, and even if you consumed it, it's only a bit dodgy to eat for people who are mould sensitive.