Shipping service DHL managed to pull a fast one on its competitors with a little help from some temperature-sensitive packaging, and has turned the whole thing into a delightfully mean-spirited commercial. The new ad, which was spotted by Gizmodo, shows boxes taped up with a temperature-sensitive foil thats completely black when frozen, but turns clear when warmed up. Those frozen boxes were sent to difficult locations through several rival companies, causing the delivery people to struggle, all while advertising DHL is faster in large, red letters right on the box. …