Are coloured bricks used by Set Designers recycled to make black bricks?
Short answer: No.

Long answer: After searching around the web and finding nothing related to this subject, I fired an email towards LEGO Customer Service asking them this very question.

Today I have received this reply:


  Dear Adam,
  
  Thanks for getting in touch with us and sending us this question. 
  
  I have investigated the case further with our LEGO® product specialist team and I received the following answer from them:
  
  No this is not true, the designers build the sets and then they have to get approved. If the set isn’t approved the bricks and sets stay out or on the designers desk, and the set is taken apart the bricks are used again.
  I hope this information will help to settle the debate.
  
  We want to make sure we're doing a good job for you, so you’ll always find the link to a four-question survey in our emails. Please tell us how we did today:
  
  LEGO Survey link
  
  Please let us know if you need anything else.
  
  Happy building!
  
  Ildikó
  LEGO® Service