Why Do Blacks Vote as a Monolith for the Democratic Party?

No, he wasn't. He was for the institution of "Racial Democracy"

One quote from Ze'ev Chafets book Devil's Night: And Other True Tales of Detroit pretty much sums up why black people support the Democratic Party (p.178):

But most black Detroiters do not measure their lives, or their city, by the yardsticks of the American middle class. [Mayor Coleman Young -- the first black mayor of Detroit; elected in 1973] Young may not have provided them with the safest streets or most efficient services; nor has he been able to raise their standard of living. But he has given his constituents something even more valuable: a feeling of empowerment and personal worth. Detroit is one of the few places in the country where blacks can live in a sympathetic, black-oreiented milieu.  

"Detroit is an environment where you can forget about being black," said Cassandra Smith-Gray, who heads the city's welfare department. 

"Empowerment." What happens when you "empower" black people (never forget that Coleman Young famously said he was "Black first, Democrat second.") is documented in the collapse of Detroit: regardless of the perilous nature of a city like Detroit, Atlanta, Gary (Indiana), or Newark under black-political control, the point is simply that blacks still control the city.

Black elected officials look out for black interests, knowing that the good for the collective is greater than the combined good of the individual. Once, when America was, well, America, white elected officials didn't need to campaign on protecting "white interests": all policies were looked at on how they benefited and built a better life (and future) for "Americans"... white people.

But this institution of "a feeling of empowerment and personal worth" to black people isn't indigenous to Detroit; it's located in plentiful amounts wherever black-control of a city has transpired (curiously, so our those unsafe streets, inefficient streets, and lower standard of living that plagues 90% black Detroit).

Richard Arrington (a Democrat) was the first black mayor of Birmingham. Elected in 1979, his memoir There's Hope for the World: The Memoir of Birmingham, Alabama's First African American Mayor basically insinuates that before a black man was elected mayor of Birmingham, the world was doomed; "hope" finally being unleashed from Pandora's Box at just the precise moment when people went to the polls to cast their votes for Arrington.

He writes (p.80) of the glories of working to create a monochromatic public labor force:

The success of the city in increasing the number of blacks in its labor force was outstanding over the next two decades. By the end of my tenure, black city employees made up 50 percent of the total labor force. By 1995, 60 percent of the city's black employees had some supervisory responsiblity. The number of black department heads during my tenure as mayor increased from one to twelve out of twenty-four department heads.  

The hiring and promotion of women also increased signficiantly. By 1999 the City of Birmingham had the most diverse labor force, both in terms of gender and race, of any government in the State of Alabama. 

"Empowerment." Augmenting "personal worth" in the City of Birmingham, for 'his' constituents. Of course, as Birmingham (and the world) prepares to celebrate the 50th anniversary of the events of 1963 - "But for Birmingham," Fred Shuttleworth recalled President John F. Kennedy saying in June 1963 when he invited black leaders to meet with him, "we would not be here today" - there is no room for discussion of what happened to Birmingham since Democrat Black-political control became enshrined as uncontested state-power in the city. 

Though blacks living in Birmingham have fared poorly since black-political control of the city emerged in 1979, blacks are still in political control.

We live in a nation where all non-whites are encouraged to play the game of "racial democracy," so that they may feel 'empowered' and a greater sense of 'personal worth' -- regardless of the consequences of their actions.

Now you know why blacks vote as a monolith for Democrats.