That's what I was going to say Cadet Corporal! A little LED at the tip would be ideal, centrifugal force makes two electrical contacts inside the blade touch.
Battery powered or even better some conducting bushes at the hub to the aircraft's electrical system, positive and negative separated front and back.
I had another idea for the people at 3M. Could they invent a material suitable for use as a paint or a sticker that converts some kinetic energy (at the prop tip) to light energy? Or use the friction to warm something up and it glows, something like that?
My first thought was convert it to sound, but the only way I could think of was to put whistles on the tips and I know this slowed my frisbee down so much it didn't make it across the river one day. Plus it would be reaaaalllllyy noisy!