Why is a layer of the subsoil so dark on this construction site?
Regarding your question:


  So now I'm wondering if that might be the "fire layer" where the excessive heat hast left its mark in the ground.


The short answer is no. The layer seems to be about 1 meter thick. No bombardment related fire can cause blackening 1 meter deep.

Now, why is it "black"? First of all, it's not black. It's dark brown. I've seen black soils, and trust me - they are pitch black. There are several reasons for it. My best guess is that it's simply moist. You know how things appear darker when wet? Same here. My hint is the floor of the site. I'm guessing it's the same material, but it's drier so it looks lighter.

There are more reasons, some concerning the organic content of the soil (for example, peat - a highly organic black soil) or the mineral content of the soil (soil composed of high iron contents will also appear black).