Yes, I was wondering the same thing. I am so fair that I have to use sunscreen everyday in the summer on any exposed body part!
I am currently using my Black Leather Piper Speedy and it is now greyish/blue with sunblock all over the front of it!
It has been pre-treated with Wilson's TLC Leather & Suede Protector, but I was nervous about cleaning it with something.
I used soap and water on a damp cloth to get it off my own black leather car seats and it worked great, but I am nervous about the softer, more delicate leather of the bag.
I use a leather cleaner/spray called Xymol, bought in the car care section at Target. I wear a lot of wpsunblock and you can see where the back of my legs have been on my black leather seat. I spray it on and clean it with a cloth.