I agree that black plastic is a HUGE help for gardeners who aren't in the deep South for these plants. I've only grown them twice, (we're not huge sweet potato fans), but got plenty of large tubers. What I used is a plastic mulch film that Johnny's Selected Seeds used to call "IRT" mulch. They now call it "solar film".
The difference between this stuff and plain black plastic is that the solar film heats the SOIL... not the plastic. It allows the heat of the sun to be transferred *through* the plastic to the soil. It works so well that you can walk barefoot on it in August... I wouldn't recommend you try that on regular black plastic!!
I use the Solar Film on all "heat loving" crops- tomatoes, peppers, melons. I use plain black plastic on cool season stuff- cabbage, broccoli, brussels sprouts, etc.
In fact, this stuff is so important to our ability to grow our own food in this climate that I've got two big rolls of it "in reserve". Yes, if TSHTF and we can't restock at some point, we'll run out... but in that case, we'd likely have 8 or 10 strong, useful, hardworking adults who would be pitching in with the work of gardening.
Summerthyme