What is this cable-joining black shell component called?
That is actually a ferrite core.  It helps remove noise from the wires that are run through and looped around it, the wires are not joined in the middle of it.  Basically they are just used for signal integrity and interference issues and can be clipped on to existing cabling if there are such problems.

Check a listing out here.  They come in all sorts of shapes and sizes so be sure to get one that is suitable for your cabling.