I use them to isolate props light, but mine are small (4"). I'm not sure how large yours are but they could be used to light some props in your haunt. You could set them on the ground and disguise them with moss or whatever to light your haunt. But if you are in a rainy area, they need to be covered and the connections covered. not to mention Grounding! But As a general rule you don't want to flood the yard with black light, it detracts from the costumes of the costume of the TOT'S. (JUST My experience)
I use several blacklights in my display every year. One I put under the eve in front of my metal garage door. I use blacklight hairspray to write a message on my metal garage door (Beware, Watch Out, etc.).
Another I put up in the entrance where candy is handed out. The TOT's LOVE to see their costumes glowing under blacklights!
Another I put in front of a huge white cotton spiderweb that I soaked in RIT whitener. It's a cool efffect as well. There's a ton of things you can use them for!
All of my blacklights are just the 4' shop lights you can get at Home Depot for around $12 and a 4' blacklight that they also sell at Home Depot or Lowes. As was stated earlier though, they should be kept out of the weather unless precautions are taken.
I use blacklight before for posters, party, and for fun. Last time i used blacklight was for my friend's b-day surprised party and everyone was wearing white t-shirt and using many different light effects such as strobe light, flashing, and disco ball and of course blacklight-big one i think it was 41''.