Who are they getting to do these blind tests? Coke, Pepsi, and RC cola have distinct flavors -- the generics do too, although I don't have enough experience to identify them. Cans, bottles, and fountains don't taste the same either. Branding and affinity certainly influence which vendor is preferred, but the drinks are not the same.
As someone who aced the Pepsi Challenge when they were around, they have very different tastes (I don't like the way Pepsi tastes at all), and I'm not the only one.
In fact, the Pepsi Challenge existed solely because Pepsi thought theirs tasted so differently.