What do you expect it to do though? Draw empty pixels? You specified a region which is larger than the texture. What is the goal?
It does this because the texture's border pixels on the right are not transparent pixels, they are blue pixels. And because the texture's import settings don't repeat by default, the right border just keeps being redrawn causing this sort of trail.
But without knowing what you really want, it's hard to tell what you need to do.