Register LED blink without reference to ground
One somewhat more complex method is to use a DC-DC converter to provide an isolated (say) 5V and operate the optocoupler input with the isolated 5V. 







simulate this circuit &ndash; Schematic created using CircuitLab

A suitable DC-DC converter is only a few dollars. You may have to add a dummy load (could be used to operate a power LED) to keep a cheap DC-DC happy. 

Do not use this method without some expert help, as Chris suggests in the comment above, if there is any possibility the LED you are connecting to is not isolated from the mains or other dangerous voltage.