I have always used black tarps for this purpose, because weeds grow under the clear stuff for me. I may not have pinned it down close enough though. Others say clear is faster. Does this have anything to do with climate variation? Does one method work better in hot, sunny climates, and the other in a cooler, shadier place?
Here are some opinions from users on this site:
In comments to user923's answer to the question 'Is it possible to kill a large area of weeds organically/naturally?', user923, as well as Joe, recommend using black. In chris's answer to that question, he recommends using black.
In trip0d199's answer to the question 'What are some natural ways of killing ground ivy (creeping charlie)?', he/she recommends using a dark tarp. In OrganicLawnDIY's comment to that answer, he recommends using clear.
In Randy's answer to the question 'How do I get rid of Crab grass in an over grown garden that will be a veggie garden next season?', he recommends using a 'covering to hide the sunlight'
In stormy's answer to the question 'What is a long-term solution to a goathead infestation?', she recommends using clear.
In Robert Cartaino ♦'s answer to the question 'How do I kill a butterfly bush so it stays dead?' he recommends using black. In my comment on that answer, I (J. Musser) recommended using black.
This is somewhat confusing. Have there been any 'scientific', unbiased studies showing that one is better than the other? I'm not really looking for personal opinions, although they may be helpful. I am looking for evidence from an authoritative source.