This stack of blue sheets is carbon paper (also known as carbon copy paper).
Before photocopiers and printers became common, people used carbon paper to make instant duplicates. You would place it between two sheets of paper, and whatever you wrote or typed on the top sheet would copy onto the bottom one.
It was a staple in offices, schools, and homes for decades. Many older generations remember the smell and the messy blue stains it left on your fingers.

