That’s all you’ll generally get with glazed white tiles; you have to color them to see the markings. Gkids on a Photo Tile
I’ve done some bisque tiles, and they turn gray where the laser hits, but I haven’t been able to get a full black. Bisque Tile Experiments
There is a company that makes tiles that turn black where you laser them, but their website is down right now: http://lasertile.com/