Usually one black batt cable jumps/connects from batt 1 neg over to batt 2 neg, then another black batt cable jumps/connects from batt 2 neg to the engine block. This series connection, like Silver said above, the two batts neg's and the block is now THE common ground.
Use either neg posts and/or the block as the return for all electrical devices....
Sometime/usually then a heavy'ish gauge (6/8/10 AWG) black wire will tap off this common ground somewhere (neg post on starboard batt on my boat) and will be ran up to the helm's fuse panel common ground buss bar. Use this ground point as well for electrical stuff return's up front.