What was the blue part of the eraser initially meant for?

Eraser, piece of rubber or other material used to rub out marks made by ink, pencil, or chalk. The modern eraser is usually a mixture of an abrasive such as fine pumice, a rubbery matrix such as synthetic rubber or vinyl, and other ingredients.

A typical eraser consists of hard and soft parts in different colors. And for some reason, we are used to thinking that the blue part, which rubs the paper harder, is meant for erasing pen marks.

The harder part was supposed to be used on heavy grades of paper such as grainy art paper, where pencil marks are difficult to remove. The soft end (pink/orange) is useful for light grades of paper which the hard gritted side would tear apart, and also for precision erasing, such as removing a light stray mark from among dark marks