What created the Blue wall was the shift in emphasis of the GOP to the the sunbelt and especially the priorities of the sunbelt suburbs, which being very religious, and favoring lesser government intervention and free trade made them decidedly less appealing to the rust belt states.
I'm not convinced this explains it. Both parties were nearly down the line free traders from 1935-2015. If anything it was George W. Bush who flirted with protectionism with the steel tariff in 2003. I do think the Blue Wall was mostly an illusion. The Upper Midwest likely would have given out on Democrats in 2004 were it not for the Iraq War.
I think it's a combination of 1. The labor market basically being continuously awesome for low-moderately skilled people from 1945-1990 (the 70's were rough economically, but that was mainly because of inflation, not unemployment) and then that system (and the congressional consensus it supported based on dividing up the surplus from all that economic growth) suddenly showing weakness in 1991 and 2001 and then breaking down completely in 2008. So it was more of a swing vote to whoever seemed to be looking out for them better economically during 1992-present than it was a solid Democratic vote, but they also tended to be very dovish on foreign policy which significantly limited Republican upside between Reagan and Trump.
