Andrew, Rich,
From what I remember reading from Bill's info, he claimed the black was really helpful for heating up the roots here in NJ, which generally contributed to strong tree growth. (We need all the help we can get.)
Also, the main reason for the plastic top (as opposed to open or bark-covered) was to keep extra water out -- i.e. the tree would only draw as much or little water from the reservoir at the bottom of the SIP as it needed. I lost a bunch of rooted cuttings due to the extremely rainy weather we had here in NJ last May/June, where my pots were just continually saturated....
Andrew -- my figs were also overwintered in the basement, and more than a couple (primarily 1st year plants grown from cuttings last year) are looking mighty dry! Many of the others have already broken bud and are leafing out, but some of them remind me of mummies! I'm really hoping that after things warm up and I put them outside, they'll plump up a bit and send out new buds or new shoots from below ground. But no matter how bad they look, I'm not throwing them out until I really give them a good chance to revive themselves.
Cheers,
Jim