why are most tarps blue?

The color of the tarp designates its thickness/durability/size. Blue are lightweight (economical) tarps. They are the most common because they are the cheapest and carried in most stores.

I doubt if there is a reason except that the company who first made them had blue dye or plastic on hand. It even became a joke, heard of FEMA blue? You will see many blue roofs in Florida after a hurricane. Silver is a better color as it will tolerate UV light better.