Making an appropriate UV-sensitive indicator would be extremely challenging. The amount 0f UV light that the wiper blades receive is all over the map - literally. It is well known and documented that UV exposure varies with latitude, but also with longitude. For instance, Georgia and Arizona share many of the same degrees of latitude, but the weather in Georgia is much cloudier and rainier than in Arizona. Other factors that influence UV exposure would be how long the car was parked inside or outside during the day and the direction the car faced when parked outdoors.
Parking the car in the shade of say a tree for example, is not as effective at reducing UV exposure as you might think. Rayleigh scattering intensity goes with the inverse 4th power of the wavelength, so UV light is the most highly scattered light from the sun. If the wiper blades can see blue sky at all, they are getting some UV exposure.
But all these factors could be swept aside and ignored if you make the assumption that UV exposure leads to degradation of the rubber and that the degradation of the rubber is the reason that wiper blades need to be replaced.
That unfortunately is false. Wiper blades are made with 2 well-defined 90 angles. I learned from past clients that this sharp angle is essential for wiper performance, but friction and erosion from dirt lead to this angle becoming rounded off over time. This is why wipers need to be replaced. This means that you cannot correlate UV exposure to wiper wear. As an extreme example consider a very rainy climate where the amount of UV exposure will be lower, but the demand and wear on the wiper blades will be very high.
 It could also have been clear too and just looked black because of the black background.
 I will have the lab here look into the chemistry of this color change and post what I find. My initial guess: it was a polyacetylene or polydiacetylene polymerization. As the polymerization proceeds, the conjugated backbone develops which then absorbs UV light.