My white Apple Watch strap is stained blue. Has been since about 1-2 months after I bought it. Any idea how to clean it?

Another vote here for a Mr. Clean Magic Eraser! I have used it to keep my white sport band pristine. It takes a little bit of time and elbow grease, but it definitely works.

You can also try a Mr. Clean Magic Eraser.

Some people have had success using Oxiclean. However, I've found the white band to never fully be white again.