a battery can have multiple ground wires going to it, that's no problem. one of the wires is going to the vehicle body. the other i can't see. they chose one of the dumbest places to put their new ground cause it gets wet there as is obvious from the rust. i would relocate this ground soon.
in your trunk, the blue wire is likely the remote signal wire and the black wire was to ground their amp
in your trunk, the blue wire is likely the remote signal wire and the black wire was to ground their amp