This is what the blue part of erasers erase
It was a common belief in our childhood that the blue part of erasers could erase pen marks and we swear by it that this became the prime reason of many ruthlessly torn pages. Turns out, the blue part was never intended to erase the pen marks, ever. Wondering what good it was other than just plaguing our childhood with the annoying mystery of a pen eraser? Well, it was actually invented to remove dark pencil marks on high-grade papers without tearing them apart while the pink part of the eraser existed only to erase light marks from light grade papers. There you go, the blue eraser part really had a purpose after all!
h/t: Life Buzz