Dark area in boshi

[Ilovekatana]

In a sword I recently got there seems to be a dark area in the boshi and I don't know what it is. For one its only visible on certain angles under light. Also the temper line/lines around it are weird. Here are some pictures of it(apologies for the quality, this is hard to take a photo of with my phone). The blue lines are a rough approximation of the temper line to show how its kind of weird(the lines might connect at the kaeri, but I don't know for sure). Does anyone know what this might be?

[Mark S.]

Just a bit of wear to the hadori portion of the polish?