I’m in the process of rewiring the bonnet in my car and came across this old post.
For what it’s worth, I am presuming that the extra black wire is used to daisy-chain two separate terminals on the connector block, which would provide additional bonding points for ground wiring.
Each block connector provides 4 connections. Daisy-chaining 2 connectors would yield a total of 6 connections for ground wires.
Thinking about the required ground connections, I come up with the following for the Left Hand side of my Left Hand Drive car:
Ground to Frame
Ground to other side of Bonnet (I.e. Left side to Right side connection)
8-Way Plug Connection
Turn Signal Ground
Fog Light Ground (?) (I’ve not seen a photo of the Fog Light option)