User blog comment:Miskos3/Fairy Tail Chapter 483: Seven Stars, Review/@comment-186.45.141.44-20160502213443/@comment-7418318-20160502214810

It's called a scars.

A bruise is a discolouration or sometimes swelling that'll go away after a short time. For example if you hit yourself with a hammer by mistake, then you'll get a bruise (assuming nothing's broken).

I'm not absolutely sure, but I think Natsu got his scar from his fight with Future Rogue.