Black Pirates are more your old fashioned Pirate, and I think their end of the spectrum fits Gunnar decently enough. They have essentially no boundaries, and do whatever the heck they want, when they want. They aren't evil per say, but they definitely aren't good either.
Why couldn't Gunnar simply be a black pirate who is interested in the possibility of a hefty reward for helping find whatever it is Shiho is looking for? It could be explained that his ship is the only one available for hire and/or he could lie and "pretend" to be a blue rogue (at least for the duration of this job) in order to put his foot in the door. This could mean that his black pirate tendencies can come to light later on, and it'd create a neat little subplot full of drama, and good opportunities for character development for everyone. Who knows, maybe Gunnar decides his time spent with the others gives him a different perspective on life, and he eventually develops into a blue rogue? Maybe through him, he brings forth temporary union between black pirates and blue rogues in order to fight against the tyranny of Valua, to stop them from destroying the world? Perhaps throughout the course of our story, the very definition of what it means to be an air pirate of either flavor is changed, due to the world events. There are endless possibilities, and I'd dearly love to see what could happen. Just my two cents. <3