Trump got only 8% of the Black vote in 2016, but that was better than Romney got in 2012. Black turnout declined in 2016.
Trump has done some outreach to the Black community and to several high profile Black celebrities. Under his watch, Black American unemployment has dropped to the lowest since they started keeping statistics. Trump successfully pushed a bipartisan justice reform bill that will benefit many Black American. He has justified opposition to illegal immigration by saying it undercuts wages of inner city poor Black Americans, who as Americans should be helped first. He also advocates an infrastructure program that will benefit Black Americans. If conditions deteriorate further in Democratic Party controlled cities, more may tempted by a “what do you have to lose?” argument.
You make a prediction:
On election day 2020, enthusiastically or reluctantly, blacks will vote for the Democratic candidate — whomever s/he may be, as most people of color will do.
Granted he will not win anything close to a majority, but I think that if the economy is still good in 2020, he could well take over 15% of the Black vote. A Democratic candidate who is overtly Socialist or Left Progressive could lead to another low turnout by Black Americans, especially if the election is overshadowed by Impeachment proceeedings or a nasty Supreme Court confirmation fight.
In any event, your statement seems far too general and simplistic: all Black Americans will not vote as a block against Trump in 2020. Most will not vote for him, but he may end up with more support from Black American voters than you think.