The darkest blue counties are the ones that have the heaviest influx of immigrants, i.e., diversity, per 1000 population.
The US Census Bureau recently released estimates of how the population changed in each of America’s 3,142 counties and county-equivalents between July 1, 2015 and July 1, 2016. Among those estimates are the major components of population change, including net international migration, or the number of people who moved to a county from another country, minus those who leave the county for another country.
BI also includes a bar chart on the top 10 counties with the most immigration. Number 1 (winner of the booby prize) is rural Jefferson County, Iowa. The Maharishi University is located there according to this article. That may explain the influx of foreigners.