The flavor was originally conceived 10 years ago as a Taco Bell exclusive (they call it a "Taco Bell Original") featuring the taste of tropical lime.
Indeed, some fans cite it as the sole reason for Taco Bell visits and it is the most requested flavor by Mountain Dew consumers.
If you're looking to stock up, the cans and bottles will only available until the end of summer, while supplies last. After that, it's back to Taco Bell for your only source of Baja Blast.
If you're having trouble finding Baja Blast, Mountain Dew has an online product locator that may help you in your search.