The blue color is used for cabin lighting positive switched. The black should be ground (negative battery). Since I see four pair (four positive blue and four black negative) I am gonna guess they are factory installed for lighted cup holders or other items that were not put on your boat. Connect something to one and see if any switch on the dash powers the circuit up. Factories install harness for most common accessories whether that accessory is included with the build or not. All four blacks should be connected to the same common place. The four blues could be also.
If you can trace the wires you may find they are not connected on either end. Or you may find they are not connected on just one end. Regardless you could use them for LEDs as long as they are unused for anything else.