I thought I heard Constantine's look was inspired by David Bowie

Nope. Definitely unquestionably Sting.

Moore describes the creation of Constantine as being drawn from a number of "really good ideas... about serial killers, the Winchester House, and... want[ing] to draw Sting in a story." Calling these disparate strands a "big intellectual puzzle," Constantine was the result of "fit[ting] it all together." Initially created "purely to get Sting into the story," by the time of the 1985 San Diego ComicCon, Moore stated that "[i]t's turning into something more than that now."

Now I'm confused, was it Sting or David Bowie who had the walk-on role in Yellowbeard?

I thought I heard Constantine's look was inspired by David Bowie

Nope. Constantine = Sting. The vampire that Constantine deals with in the Hellblazer issue that Gaiman wrote = David Bowie.

was it Sting or David Bowie who had the walk-on role in Yellowbeard?

According to IMDB, Bowie.

The vampire that Constantine deals with in the Hellblazer issue that Gaiman wrote = David Bowie.

Ohh, maybe that's where I'm confused.

Oh wait, I'm mixing things up again. Gaiman didn't write the Constantine vs. vampires issue.

I've heard it's not politic to say so, but since I didn't read the comic, I loved Keanu as Constantine. I also don't care for Sting outside of his looks, though, so there's that.

Amy, take my word for it. You're unexposed.

The character doesn't have anything to do with Sting outside of his looks, as far as I know anything of Mr. Sumner. He's irretrievably bitter and wry and funny and scarred in a way that...well, maybe Keanu pulled it off in the second half of the movie, but I sure didn't see it before I crapped out.

The movie wasn't a bad movie. It just wasn't John Constantine.

It's one of the comics I'd like to pick up. The list is getting longer.