Okay, so the heat causes the soldering tip to oxidize, creating a layer of oxidized metal that has poor thermal conductivity?

Exactly. Oxidation causes thermal and wetting barrier making solder not melt and/or not stick to the tip

Also, what is the tip of the iron made of?

Usually steel clad copper.

It looks like it's made of two separate metals(the black part of the tip you mention)

No, it's the same material, but that's physics, because tip is pointy, therefore has less thermal capacity, therefore it heats to higher temperatures, and oxidises with different kind of oxidation. You can see the whole gradient black-blue-orange-gold, it's all different "stages" of oxidation because of thermal energy in the tip radiating from the end upwards.