Definition of blue wall:
(n) :
(US politics) A group of states known for consistently voting Democratic in presidential elections from 1992 through 2012.
(n) :
(US politics) The three battleground states that were dislodged from the 1992-2012 group in 2016: Pennsylvania, Michigan, Wisconsin.
(n) :
(UK politics) A group of Conservative-leaning parliamentary constituencies that are seen as shifting away from the party.
(n) :
(law enforcement politics) Alternative form of blue wall of silence. [(idiomatic) Strict secretiveness maintained by the members of a police force with respect to information which might be contrary to their interests, especially information concerning questionable police actions.]