This is because you need to add a padding to your tiles.
This is a pretty common problem and you are not the first to encounter it. Basically due to rounding errors when scaling and panning around, sometimes you will render the area "between" two tiles, which will result in nothing being rendered -> black background colour comes through.
You basically need to use some tools to add the padding to your tileset. In this forum thread I explained how to do it.
There is also one more questions regarding this topic on stackoverflow here.