If you're looking for manga similar to Black-Box, you might like these titles.
Taichi is a fat teenager and a horrible embarrassment to his little sister, Mika. She saves her own image by refusing to acknowledge him in public, but that won't work forever. In order to solve the problem, she enrolls him at a boxing gym, hoping the exercise will make him lose weight. At first, Taichi is very hesitant about the whole idea. Soon, though, his trainer Kurata's tough encouragement makes him feel like he might actually be able to become good at this sort of thing. But while the boxers at the gym accept him as a fattie coming in to jump rope and hit punching bags, will he be able to get them to accept him as a boxer-in-training? And does he have what it takes to become an actual boxer?
The story revolves around a 202-pound, 18-year old girl named Okome Akino (whose name is a pun of "autumn rice" ). After Okome enters a sumo match and wins against a male amateur fighter by a landslide, a trainer named Akane scouts her, and she enters the world of women's boxing.
Goshima is a boxer who has held the world championship title for years and never once lost a match - thus earning himself the nickname 'Zero'. His coach, Araki, describes him as 'a flower that blooms only within the ring'. But with his 30th birthday approaching, everyone begins to question Goshima's worth. Thus he decides to look for the ultimate opponent to give him his final match.