What is the 'blue wall' in US politics that Donald Trump broke?

'Blue wall' refers to 18 states & District of Columbia whose electoral votes often go to Democratic presidential candidate. The 18 states are California, NY, Illinois, Pennsylvania, NJ, Massachusetts, Michigan, Washington, Wisconsin, Maryland, Minnesota, Connecticut, Oregon, Maine, Hawaii, Delaware, Rhode Island and Vermont. Trump broke the 'blue wall' by winning Pennsylvania, Michigan, Wisconsin.