I brought one of these amazing electric bug zappers that looks like a tennis racket to the beach with us in August. It works very well killing mosquitos, fruit flies, and other small bugs and will stun flies but not necessarily kill them so you have to make sure they're dead after they fall to the ground. It may work with black flies. It's fun to use in a creepy way. I have one at home and one at our place in Florida. https://www.amazon.com/gp/product/B01DVWV4ZI/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1
As someone else said, Cactus juice may work, too. I've seen it at Seacrest Sundries and a lot of it at the Ace on 98 going toward Pier Park.
As someone else said, Cactus juice may work, too. I've seen it at Seacrest Sundries and a lot of it at the Ace on 98 going toward Pier Park.