What are the blue lights in businesses?

This has been puzzling me for years, and google isn't helping. In lots of businesses now you'll see these boxes hanging on the wall that have blue light coming out of the top of them. The light is directed by a sort of barrier towards the wall that the box hangs on. Sometimes in restaurants you'll see them in the kitchen. Usually if you go into, say, a Target, they'll be near the entrance, or next to the hallway leading to the bathrooms. What do these do? They can't possibly be there just to provide light, cause there's only ever one of them and the light is always much bluer than the rest of a room's lights. There's something medical looking about them. My best guess is that the blue light does something to purify the air? That sounds weird too though. I've concocted a bunch of crazy conspiracy theories about these weird lights, so now I need to know what they actually do!