Bourbon is a type of whiskey. Because the regs say it must be made "from a fermented mash of not less than 51 percent corn," you can't get there through the back door by mixing different whiskeys which, collectively, add up to more than 51 percent corn. It has to be one whiskey made from a mash of not less than 51 percent corn and "mixtures of such whiskies of the same type." Mixtures of whiskeys of different types are some kind of blend.
I love it when you're a strict constructionist. :grin: