I have not seen the BlackSky rail in many years. Once the 8020 extrusion was re-purposed for rocketry, no one locally ever bought the BlackSky rail. If I remember correctly, the BlackSky rail was single sided too.
The Black Sky rails I've seen are single sided, and some of my rail buttons don't fit them. I have no idea what they were originally for. Here is a 1999 article on RP about them: