I'm formerly a blue belt in mixed martial arts with all chevrons. Examples are Karate, Judo and Aikido. Been practicing for 5 years and that was a few years back. And self taught in Tae Kwon Do and Muay Thai, not all of it. I really need to back into my fighting form soon.
If I had a choice of another style, I pick Ninjutsu and Dragon Kempo. On the Ninjutsu, not sure on the specifics I would choose.
Currently a white belt right now in Taekwondo, learning the taegeuk form 1 is a pain, but I'll get it down eventually.
All branches of Ninjutsu are very fun to learn. The training is hard, but it yields great reward. I specifically have learned the most about are Kenjutsu, Shinobi-iri, Kusarigamajutsu, Bojutsu, Taijutsu and Naginatajutsu. All of these have helped a lot with my core strength. Strong abdominals are must (at least that is what I was told).
I recommend Kusarigamajutsu. It is challenging, requiring years of practice and self-dedication. In the end, it really hones your body into a force to be reckoned with. Really works out the triceps, forearms and lats. If your not up for something of that caliber, I would say Taijutsu. Only because it is the most practical and you only need your hands to use it to its full potential.
Like most martial arts, it's best supplemented with an hour or more of cardio. But being a blue belt, I'm sure you already knew that.