Black plastic is considered bad because they will heat up too much in the sun. Half of the problem with cold is the rapid hot-cold-hot changes are very hard on the dormant wood. Also it can cause the trees to bud prematurely in the early spring.

This year I am trying some aluminum-coated insulation. It is making it 5 degrees cooler when its sunny out, and 5 degrees warmer at night. Since the aluminum is on both sides it reflects heat away during the day and back in at night. We'll see how this works in a few months. I still need to pack more leaves in, I only got about 2' of leaves packed in and it is a lot more than 5 degrees warmer under the leaves. The leaves need to be packed over a period of weeks because they settle.

You also should have done the winterizing earlier, this year the cold hit around Thanksgiving time.

Scott

Another thing you might try is stringing some christmas lights (or to be PC "holiday Lights") around the tree. When you hear that the temperature is planning on dropping down way in the teens, or heaven forbid..negative numbers, plug the lights in for the night. In the morning, unplug. I was told this by someone. I was going to try it on my own fig this year..but after I put the hay around for insulation, I looked down, and saw the lights, still in the box...oh well, If she dies, at least I remembered to take som cuttings this year.

Please take into account that I'm in Italy where there is not problems about frozing of figs (I'hav'nt experience).
But for first protect by burlaps basis and roots. If the temp. is to limit the esternal tree die, but the lower part can survive., and become more resistant.
No black burlaps only thick leaves and straw.
Microclimate is important.
Less wet climate (drougth) is better.

Second Part:
Cold resistance is a lot dependent from variety, there are varieties more cold resistant.
In general also dimensions are important, dwarf tree (bush shaped) may be bended at the ground and than buried under soil, straw, and burlaps; (hystorically done also for vines in colder russian regions).
Adult tree may be a lot more resistant.
Quite a lot of cactus I think that location on dry ground (slopes ect) dehidrate the wood and give it more resistant to frosts.

http://www.italianfigtrees.com/blog/index.php