I THINK I'm looking at the same wires on my "62. The brown with the female connector coming out of the harness goes to two browns on a male connector, and they go to the horns. The blacks in that area seem to usually go to ground eventually, often through switches. I suspect that the black you're talking about goes to the horn rim on your steering wheel, where it grounds, i.e., the horns are always life, and sound when grounded. Caveat: The wires are a bit discolored and sort of hard to follow with the clutter (plus the heater air intake) in that corner. So, there is a guess element on my part involved here.
Bob S.