im not entirely sure but i thought i read something about that being a censorship thing in japan xD
something about if you can't see the whole cock, they its not a fully mature (rated M xD) thingy. so if you only hide a really small spot, like with such a black bar, its fine the way they see it xD
or it had to do with even selling those manga officially xD
as i said, im not sure about it though ^^
EDIT: slow post fail thanks to my UBER slow internet connection >.>
yep, just what Rob said
