(a) fewer black people are murdered when federal executive power is held by a member of the democratic party, or
(b) Trump specifically controls a lynch mob, or death squad
(c) Trump inspires and motivates, or incites, a pre-existing lynch mob, or death squad(s).
I think it may be useful to recall that what are often referred to as red states and blue states are actually quite similar in color - only a very delicate shade different in hue (maybe a few percent) and this does not change very much from one presidential election cycle to the next.
Yes, a lot of rural whites vote republican, but I recall discussion during the Obama campaign in 2008 of Obama signs in the windows and front yards of known white supremacists in Indiana.
And the cause was economic, according to the thinking at that time.
Politics has, it seems, many shades of gray, and economic justice may be a place to start looking for a robust explanation. Obama had an opportunity to introduce financial correctives, during his first two years when the democrats held the legislature also. Opportunities like this don't arise too frequently in the US system these days. Obama had donors to consider, and since citizen's united, both parties do now infinitely more.