See the blue on the side? It's not on the pages.

I think this is fine sort of like a remainder mark. You might be able to lightly sand it off too. I use the white side of an emory board at times for little marks.

In general, pen, marker, or other ink type "stains", including writing, are OK on the edges of books.

What is not OK is dirt or non-writing-implement stains (mystery stains).

Although most mystery marks that don't penetrate the paper on edges of books can be removed with sandpaper. (I know it sounds like it would tear up the page edges but it doesn't. You can try it on a trash book first) Just squeeze the pages together and rub with sandpaper. I've been able to restore more than a few books back to postability with sandpaper.