Ideological party switch. The ideologies of the two main parties diverged in the 1960s. The Democrats became the party of centralized government and government spending. The Republicans became the party of small government and individual liberties.
(I know that both parties are prolific spenders, just pointing out the ideological differences)
As the parties started redefining themselves through those visions, the Republican party started becoming the party of the pro-life movement, gun rights advocates, and pro-Christian heritage. The Democrat party started becoming the party of pro-choice, gun control, and post-Christian modernism.
By the mid-80s, most of the pro-life, pro-gun, Christian Democrats in the south felt betrayed by their party and switched (see Zell Miller). Many of those who were New England Republicans are now New England Democrats for the same reason.
As for blacks switching to the Democrat party, the late 60s is where you want to look. In the early 60s, the Republicans pushed through the Civil Rights Acts (granted, Kennedy was a firm supporter, but most Democrats were not). But Johnson saw that he could essentially buy the black vote by creating a welfare state that would be self-sustaining. His ‘War on Poverty’ and ‘New Society’ initiatives poured money into the poor black communities and the vast majority of blacks switched party loyalties.
I wouldn't be so sure of that. True, some New Englanders who were originally Republican were becoming disaffected with the Republican party as early as 1872 (the Liberal Republicans) with a big break coming in 1884 (the Mugwumps), but I doubt the Calvin Coolidge types ever became Democrats. New England was swamped by ethnic Catholic Democrats and I think that's mostly what prompted the initial change. Now of course most New Englanders are crazy, with nothing in common with their great ancestors.
Another one was former Texas Senator Phil Gramm, who started his political career as a yellow dog Democrat representative but switched in the early 80s.
BTW, he's about the only one I know who switched honorably, as he switched parties in the middle of his term and resigned his seat to run in a special election as a Republican, saying that it wasn't right that people who voted for him as a Democrat shouldn't have another opportunity to vote him up or down.