Go with a textured blue stone. There are many different stone options available you can use both artificial and natural. I've seen colored pavers or stained concrete stones that give the appearance of slate.

Also, don't just lay them on the surface. You should embed them, making them level with the ground.

Is this for a path or patio? In any case you are going to have to cut into the slope to level it off for the laying of stone. Also what is the climate like....does it snow, get heavy rain? Finally don't do anything where you might be held liable or accused of improper workmanship because of somebody getting hurt. If someone does get hurt your client will forget all about that they told you to lay down the stone that way.