Building a High Voltage Rectifier with a continuous negative output
The frequency for the input is 60 Hz and is provided by a X-ray
  transformer that provides adjustable voltage from 0-70 kV AC


Some more modern X-ray machines generate a high frequency (10 kHz to 100 kHz) variable AC voltage of several kV magnitude and then use a Cockcroft Walton multiplier like below: -



This both performs AC to DC rectification and, each stage need only be rated for the peak voltage from the AC source hence 10 kV rated diodes and capacitors can be used.


  I was curious how I could build a rectifier that could take 70 kV AC
  and 10 mA to put out a continuous, negative DC voltage with minimal
  ripples.


If you can't find diodes rated at over 100 kV DC then a good option is to scrap the X-ray transformer method and use the multiplier shown above. Minimal ripple is meaningless but if you are driving a tungsten target X-ray tube then a few volts p-p isn't going to be a problem.