The blue bar isn’t something that’s related to the Pokemon. Instead, the blue bar (called as the energy bar) is the one that depends on the move you use.
Powerful moves, that hits very hard generally have a single solid bar, which means, it requires more energy and time to fill the single solid bar, but in return, it hits harder. On the other hand, the small bars does significantly less damage but fills up faster and can be used more number of times in a particular time period.
You can refer to this Pokemon GO Wiki for more info.