Question

Name the agent that makes the blackboard black

Answer 1

these boards are made up of dark grey sheets or black slates

Answer 2

A blackboard is also called a chalkboard. It is made of thin sheets of dark grey or black slate stone which is smooth. It's surface is used for writing and can be reused again and again. People write on it with a calcium sulfate or calcium carbonate stick which is known as chalk. 

The blackboard was first used by James Pillans in 1801. He was the headmaster and geography teacher in the Old High School at Edinburgh, Scotland. He hung a big piece of slate on the wall in the classroom and used it for teaching.

The blackboard does not reflect back any color so it is black.