Does a soldering iron tip consist of multiple parts?
The delineated part that accepts solder is the part you use. The rest is just there to transfer heat to the working tip. It is good that it does not accept solder, because that avoids having stray blobs of stale solder hanging off the tip that aren't what you are actually trying to work with and might make a mess.

The un-tinned part will get dark and ugly looking with use. This is normal and harmless. I wouldn't expect to see necessarily that dark a color on a new tip, but it doesn't look unreasonable and your attempts to tin it might have contributed (burnt flux?).