What is it? (Are they?)

I have a new to me 2003 Chevy S10 4.3l
6 cyl Ext Cab LS with 89k on it. I'm
planning to trail a small fiberglass
camper (~1000 lbs empty). Ive seen the
kits for adding the wiring, as well as a
hitch. Looking around under the rear
end I see a large wire connector with a
blue cap/cover. (See the picture, its to
the left.)  I also see a round black box
(to the right in the pic) which is attached
to the rear frame. (Note: the tube in the
center of the pic is the rod that
raises/lowers the spare tire, as it comes
thru the bumper.