It's probably from a set which normally had a dark grey spear. Maybe the stock of spears used to make up the sets ran out and a few hundred more were created, but the colour wasn't matched properly. Maybe the dark grey colour was called for, but perhaps an injection moulding operative thought black was near enough!
There are other examples of sets containing slightly different coloured parts in different examples. For example, Color set
3624 is shown on the box and most pictures of real examples of the set as having a white saddle, a white shield and a chrome breastplate, but examples do exist of sets with a black saddle, a black shield or a white breastplate. Black saddles and shields are found in several other sets, so they would be readily available to use when the white ones ran out and the time and money involved in creating more white ones could be avoided. With the breastplate, I imagine it would be much easier to create white ones than chrome ones if the stock of chrome ones ran out. I believe that breastplate, in white, was not used in any other set, so it would have to be created specially.
Most sets have 'alternative parts' which were probably replacements for parts where stock had run out. Take the famous set
3666, for example. At the bottom of the Playmodb inventory page are 10 'Extra parts not include in inventory.' Most of these are identical to other, included parts. But 30077100 'Keys on keyring' is gold, not bronze. Perhaps another example of less-than-exact colour matching?
