Hi Rocky, Try a well stocked hardware store. Look for some silicon
caulk, It usually comes in a tube about 10" long & about 1 1/2" in
diameter. There are several colors available, but I don't know if
black is one of them.
One of the colors is a clear caulk. Possibly lamp black or some
other dry black pigment could be mixed prior to molding that'd give
you the desired finish.