From an outsider perspective, blacks in the US vote Democrat due to the nature of the country's nativist politics.
Elections are not a game of addition, but substitution.
Take France for example, a country with a similar demographic makeup in percentages between black-white-arab (replace arab with mexican in the US).
The centre-right party of Nikolas Sarkozy finally got the black/arab vote by promises of industry deregulation, in the subsequent election he began losing his more hardcore nativist supporters. Notice I'm not saying rightwingers, but nativist, as nativists put race, culture, and ethnocentrism above economics and social policy. They went to vote for Marie LePen's National Front. The NF is essentially a quasy communist/socialist party, why would right wingers jump there? Simple, nativism.
It's similar in the US. The Republican party mopped up the disgruntled southern white vote after the civil rights push of the 60s.
The Democrats didn't overnight become to party of social welfare in 1960, they were the party of social welfare and feminism for the past century.
The Democratic party platform in 1920:
http://www.presidency.ucsb.edu/ws/?pid=29592
It sounds very similar to the Democratic platform of today. Higher taxes, push for women's suffrage, battling the high cost of living, investment in infrastructure etc. More or less the same platform as today.
And that was the party that southern white voters happily voted for since the 1890s.
If it was a Republican president that signed the civil rights bill (it was after all a completely bi-partisan bill), and had the southern strategy never materialised, today on this forum blacks would be getting banned over being hard-headed supporters of their Republican party agenda, and many white dudes would be pushing the democratic leftist agenda. Listening to some of my European compatriots talk the good talk in respect to having higher taxes, more social equality, and even better women's rights, then turn around and spew some of the most hardcore nativist/racist shit will make you realize that economics do not dictate political affiliation for many multi-racial countries, nativist affiliation and emotion do. A bad combination.
Look at a country that doesn't racially troll its electorate and play nativist politics: Canada. The conservative party happily has 10% lead over the liberals from "visible minorities". The term assigned to non white, non aboriginal people in Canada, which make up roughly about 20% of the Canadian population, Chinese, South Asian, Black, and the others, in that order.
http://www.ft.com/intl/cms/s/0/9310918c-...z2vO0JT2Yc
The retrospect, the reason why blacks (and most minorities really) in America vote Democrat is just circumstantial. If they switched over slowly to the Republican party, which in my opinion will happen within the next few decades, there will be whole-sale transformation of the electorate landscape. Hopefully by then Americans can get over their nativist politics so that we don't get racial parties springing up by malcontent voters who'd rather not be voting by association to a party supported by people who don't look like them.
Here's hoping.