Light brightness and colour can be keyframed like any other light attribute, so building a light that flashes is pretty basic. A better way of controlling this would be with a script, since this would allow you to adjust the flash timing.
However, lights themselved are not rendered, they just affect objects around them. To build a weapon flash effect create an object in the shape of the flash, give it a strong emissive value in a bright yellow or orange colour, and keyframe (or script) its 'Visibility' controller to match the flashing light. With both a light and a glowing object animated to 'flash' in unision it appears as if the glowing object is illuminating the surrounds.
Place the light in the same physical space as the flashing object, so the illumination looks right, but make sure you uncheck the 'casts shadows' attribute of the object, so it doesn't block the light. A little experimentation and fiddling of colours and other light settings will be required to make it look right.
Having a couple of slightly different shaped 'flash' objects, triggered apparently randomly, will give a more realistic appearance to the effect, since no two flames (or plasma flares) will be identical.