Have you applied any car wax to the car in the area near the wipers? That discoloration could be caused by exposure to the elements, but sometimes discoloration can be caused by car wax or sealants that leave a resideue on rubber and vinyl.
For any type of rubber surface, including wiper blades my go to product is Mckees 37 tire and rubber restorer. You can also use Meguiars D156 synthetic spray wax on almost any car surface including rubber and vinyl. It will bring back the dark color and leave behind some protection. If you don't want to buy the Meguiars D156, which comes in a gallon size for professionals, you can get the same product Meguiars Ultimate QuickWax, which is the consumer version.