Greg, I do believe you're right about clear plastic. Down here it is necessary to solarize the soil regularly for nematodes. All the commercial farmers must do it so it's well documented. Clear plastic is the recommended stuff, for most heat generation as long as you seal the edges down well. But I can tell you from experience, the cheap stuff disintegrates very fast in good sun. Six weeks is also the normal timing, best started in early June to take advantage of the hottest sun.
Obviously, black also works as Tom's experience shows but not as well. Think of a closed unventilated greenhouse in the blazing sun all day.
