There are only a few of these in Hawai'i. This one is located along the road to Hana at the Waianapanapa State Park. You have to get there early though because the park fills up quick with people. That's partly due to the fact that they allow camping here and it seems the camping sites fill up quickly, especially on the weekends.
You can't tell it but the beach was full of people when I took this shot. There was one end of the beach where the water wasn't as calm as the rest of the beach so the people were shying away from it. By carefully zooming and cropping I was able to get a shot with no one in it.
The "black sand" is technically not sand. It's pulverized lava rock that has been made smooth over centuries of wave action. This beach is relatively young, just 300 or 400 years old I think, and that's why the lava rock or "sand" is still there instead of the golden sand you normally see on a beach.