Just my opinion, but I would not worry about how the back looked. If it is really bothering you here are two options that do not involve removing and tacking off and replacing the black thread. Try switching to a dark gray thread instead of black. Since it is in the ditch in the front you may be okay, and it won't be as stark as black on the back. Second option, even easier, get a Pigma Micron pen and color over the black thread on the backside. It won't disappear, but again it won't be as stark. Those Pigma Micron pens have saved many of my boo boos.