The blue part of the eraser is used for erasing pencil marks on grainy art paper, hence the rough and grainy texture of the eraser. It would require you to use more force to erase something written on these rough paper if you're using a normal eraser (the pink part). The bue eraser makes the task easier.
Not sure where the logic of erasing pen ink came from, but at least now you can stop damaging the pages of your exercise books now.