OMG! I wasn't aware about it.
I was just shocked to read the fact that the blue part of the erasers is not actually used for erasing pen marks. Well, we all grew up thinking that they erased pen marks and many of us probably tried it too and ended up making a complete mess out of it. At that time, we didn't realise what was its true use but seems like we have an answer now. Here's what the blue part of the eraser is used for.
How wrong I was!
But here's the difference between the red part and the blue part.
The red part is actually the delicate part of the eraser so it can be used to erase pencil marks on light paper that will tear easily.
The blue part of the eraser seems to be slightly tough in the making material, so it was used to erase pencil marks on thicker paper without tearing it.
This question has been bugging me like crazy!
But people started using it to erase pen marks as well. And it seemed to work too.