Like many users, I encountered the “black-screen” problem when installing the Nvidia drivers on Ubuntu 14.04.
I’ve tried many things, none of them worked.
According to the Ubuntu Community, the only solution in my case was to upgrade to Ubuntu 16.04. I did it and it solved my “black-screen” problem.
Unfortunately, there are currently no version of CUDA, Torch and DIGITS for Ubuntu 16.04 (CUDA: 15.04 max and Torch/Digits 14.04 max).
FYI, it looks like CUDA 8.0 does have a Ubuntu 16.04 target. However, I’m having difficulty getting it to work properly with libraries (Theano and TensorFlow) so your mileage may vary.