You're making this more complicated than it needs to be. That's precisely what a transistor will do.
In the circuit above, the photoresistor pulls the base of the transistor down to ground when there is light present. This turns the transistor off and interrupts the circuit's connection to ground, thereby switching the circuit off.
When it goes dark, the photoresistor's resistance goes high and the base is then pulled up to the supply voltage via the base resistor. This allows base current to flow, which then turns the transistor on. The transistor supplies a ground to the circuit it's switching when turned on.
Once it goes light again, the base is pulled down to ground, which turns the transistor off and interrupts the ground to the circuit, switching the circuit off.
The only thing common to the controlling circuit and the controlled circuit is the emitter of the transistor, which is tied to a common reference. This will not affect circuit operation.