I second that.NickButler wrote:I think I've heard they just last longer. I just know, mainly for sports, people advertise it if it was signed in blue.
Black sharpie, as it fades away with time, will change the colour from very black to brownish, bronze or something like that. While blue will stay blue over the time, making the autograph stable and more valuable.
Professional autograph dealers know that, so some celebs think you are out there to make money. It is better to carry more sharpies with you just in case.