At present, eraser tool fills a circle of white color which is not the correct way. Instead of filling white color it should provide transparency on the drawing to display the background. i.e set the alpha channel to 1 at that particular pixel.
Improvements --> Brush and Eraser tool will share same "tool options" panel and while eraser tool is active, different brushes will be used to erase the drawings in there respective pattern.
Tool Options will have another slider for changing the alpha channel value of eraser and brushes while Eraser tool is selected.