I just want to add that even not every BL manga has the same characters. How "uke" or "seme" behave is not etched in stone, neither are the heigh rule or hair colours. Sure, several popular works follow the known stereotypes, but just as often you just find two guys, one of them happens to top and one of them happens to bottom, sometimes the role changes with other characters, and sometimes you can also find pairings that switch. It's not one way to rule them all. ;)