If I want to blink a light, I usually animate its power. For example set power in Frame 1 to "100", set a keyframe. Go to the frame where you want to switch it off and set the power to zero. Add a keyframe. The default will cause a gradual transition from 100 to 0 for the intensity. To blink on/off you need to adjust the time spline. Go to the timeline and select the light in the left panel. Under the light you find its properties. Select power to see its keyframes and curves/time splines (you might have to switch them on). Edit the time spline so that you get a step like transition or load one of the pre-defined time splines. See my screen shot above.

TD