The map was put together by Nate Cohn and Toni Monkovic of the Upshot feature of the NYTimes, and the two of them have a great discussion about what this all means. Here's a snippet:
Toni One way to understand Blue America is to follow the water. The oceans, the Great Lakes and the major rivers were obvious places for small settlements to become major cities. And these big metro areas are where Democrats dominate.The population of rural counties as a whole has been declining, and metro areas are growing. Over time, this will help Democrats, no?
Nate The Democrats do tend to do best in metropolitan areas. All of the major demographic and cultural changes that have helped the Democrats over the last decade are concentrated in these diverse and often well-educated areas. In this election, I’d guess Hillary Clinton will fare even better in metropolitan areas than President Obama did.The red parts of the country tend to be rural, predominantly white, less-well educated--Trump country. The blue parts in the southern states reflect counties that historically had plantations with large slave populations and they are still predominantly African-American. Blue areas in Texas, New Mexico, and Arizona are counties with high percentages of Latinos and/or Native Americans. Blue counties along the two coasts reflect the better-educated urban populations--these tend to be Clinton country.