It sure can be, especially when your lungs feel like they're going to burst and the surface appears to be minutes away. I had to dive to 53 feet for my free diving certification and it's very dark down there (seemingly more so than when you have tanks on your back:).

For anyone free diving at that depth, I'd encourage you to ensure there's always someone nearby that is capable of rescuing you.

Shallow/deep water blackout[1] is certainly dangerous with a buddy, but it's deadly if you're alone.

[1] http://divewise.org/science/physiological-causes-of-freedive...