Change in Total Turnout, 2012-16

[MarkD]

I'll add this info to your discussion. I have finished analyzing the number of votes cast in my region - St. Louis City and County.
I live in the 1st Congressional District. The population, according to 2010 census, is roughly 50% black and 47% white. In the 2012 election, I have counted 351,065 votes cast for President; 79.87% of that for Obama, 18.89% for Romney, and 1.24% for others (Johnson and Goode). But in 2016, the number of votes cast, according to my count, is 320,559. That's a drop of 8.6%. Clinton got 76.81%, Trump got 18.76%, and 4.43% were for the others (Johnson, Stein, Castle, and write-ins). The drop in the number of votes cast for the Dems was almost 34,000; the drop in the number of votes cast for GOP was about 6,000; and the increase in the number of votes cast for others was about 10,000.
Within St. Louis City and County, I have also been calculating the presidential votes within each State Representative District, of which there are about 36. Districts 67, 68, 73-79, and 84-86 are the ones that are predominantly black. Those 12 districts are adjacent to one another, and collectively that region is 70% black. That's where the vast majority of the decline in voter turnout occurred. I calculated that in 2012, those 12 districts cast almost 210,000 votes. But in 2016, the number declined to a little over 182,000 votes -- a drop of 13.2%. There was a decline in the white-majority State Representative Districts within the 1st Congressional District too, but no where near as drastic.
I'm sure there is some population decline occurring -- out-migration -- but it surely is not as high as 13.2% in just four years.