Pretty standard tilemap issue in cocos2d.
The quick fix is called "fix artifacts by stretching texels" in ccConfig.h but it's a hack-ish workaround with aliasing side-effects, particularly visible when scrolling slowly or zooming.
The actual fix requires to set all positions of the tmx node, its parent and all grandparents to exact pixel positions. On a non-Retina device this means casting the x/y coordinates to int, on a Retina device it means rounding to the next-nearest 0.5 coordinate (ie 10.3 becomes 10.5, 10.8 becomes 11.0).
PS: The KoboldTouch and Kobold Kit (Sprite Kit) tilemap renderers don't have this black line artifacts issue.