Closing the Derivatives Blended Rate Loophole Act - Amends the Internal Revenue Code to treat all gain or loss with respect to a section 1256 contract (i.e., any regulated futures contract, foreign currency contract, nonequity option, dealer equity option, and dealer securities future contract) as short-term capital gain or loss (currently, 60% of such gain or loss is treated as long-term capital gain or loss and is thus taxed at lower marginal rates).