NO! that black box with all the red wires is B+ power straight from the battery. Do not ground to that!!!!!
There is a good grounding point right next to it- all the brown wires in this photo.
edit again - there is no negative in that black box, it is all positive. the fusible link is a little further down, kind of a lump on the red wire. you can take power direct from the terminal in the photo and put any fuse on it you want. 60A is a lot tho. and you don't need to fuse the ground.
fusible link -> http://www.bimmernut.com/forum/showthread.php?t=39960