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In the Fujian area of China, there exists the blue tiger, a suspected coloration mutation of the normal Bengal tiger. It is called the "Maltese Tiger" in some circles. Blue cats due to coloration morphs certainly exist, but no large population of blue cats has ever been observed. However, the idea of a Blue Tiger population is definately possible, especially due to the blue allele that exists, or at least existed, within the normal tiger population. Whether any blue tigers still exist is in question, though it is almost certain that there was once a population of these creatures.
Image from http://upload.wikimedia.org/wikipedia/commons/f/f7/Maltese_Tiger.jpg