Can I paint and or repair the back side of a wall that is on both our property lines. The back side of this wall is very unsightly with exposed cinder blocks half painted.

Does the person who shares this wall have a problem with it? I don't see why you could not paint it and why they would care. It would be good for the wall. Use block filler if it is unpainted masonry.

Is this a wall that runs alongside and between both of your homes? If so, then it is likely that only one of you owns it and is responsible for it. Usually, walls of any sort between properties belong to one or the other party. I would suggest seeing who owns the wall before proceeding with any idea as to what to do. If course that does not preclude the fact that you are neighbors and you might come to some sort of mutual agreement as to what needs to happen to the wall no matter who owns it. Sometimes, people even build things over onto the property lines of another through a mutual agreement that what is being done is for the benefit of both parties.

As we both live in the city, his front door is on one block and mine is on another. I don't think they have ever sen the back of the building since it was built. I don't think that they really care because of how really bad it is, but I just redid my while backyard and this wall makes the whole thing look awful. The wall is right on the property line. I'm looking at the point of view where, say I lived in the subabarbs and I asked the neighbors to go half on a property line fence and they refuse andthen they start to get nasty with us... I would then install the fence myself and then paint circus designs on the back side of the fence.. as long as the fence is totally on my side, they can't touch it -right? Now, if the fence was right on the property line, can they paint it without my consent?