I've heard from some mac users that the game secretly "loses focus" sometimes after starting it, so it looks stuck on a black screen, but it's actually just not focused and 'paused in the background'. Try forcefully giving it focus again.
If that doesn't do it... I admit I'm not sure. The mac demo build is quite a bit outdated by now compared to our main game version as we've redone a lot of systems to ensure the Mac build runs as smoothly as possible. We'll eventually put a new demo build up, but we're focusing on our Beta Backers build currently.
We have a new build and I believe this issue should be resolved in it. We're planning on putting it up soon. It contains about 6 months of various fixes from our Beta Build and should be looking a lot better :)