What was the first year in which a majority of blacks voted for a
democratic instead of a republican candidate for President?  [I'm
guessing that after the 15th amendment African Americans began mostly
voting republican, and in recent years most vote democratic.  When was
the cross over?]

Hello.

Historians cite the election of 1936 as the crossover.

See:

"The 1936 election...  
Another historical thing that happen was that for the first time in
the history of the U.S.A. the majority of black voters, who were
allowed to vote, voted for a Democrat.  Until this election, blacks
had always voted for the party of Lincoln, the Republican Party,
because it was under the Republican administration they received the
right to vote."
source:
Kennesaw State University: 1936 Roosevelt v. Landon
http://www.kennesaw.edu/pols/3380/pres/1936.html


"By the time of the 1936 presidential election, millions of black
voters began switching from the Republicans to the Democrats because
of their strong support for Franklin D. Roosevelt and his New Deal
economic programs."
source: AfricanAmericans.com
http://www.africanamericans.com/BlackVotersandtheDemocraticParty.htm


"...For nearly 75 years after the Civil War, the black vote was
strongly Republican in memory of President Abraham Lincoln. Black
voters were the most Republican and white Southerners the most
Democratic voters in the nation for essentially the same reason:
memory of the Civil War.
  Even in the depths of the Depression, a majority of black precincts
supported Herbert Hoover in 1932. The Depression permanently changed
things among Northern blacks: They voted for Franklin Roosevelt in
1936 and have remained in the Democratic column ever since."
source: InsightMag.com
http://www.insightmag.com/news/2004/03/02/Politics/Analysis.Black.Vote.Key.To.Kerrys.Charge-609460.shtml

" Nonetheless, in 1936 African Americans rallied around Roosevelt and
the New Deal. This support represented an electoral shift of historic
proportions: for the first time since emancipation, a majority of
black voters cast their ballots for the Democratic Party."
source: Africana.com
http://www.africana.com/research/encarta/tt_644.asp

-----
search terms:
democrats "black vote" republicans 
democrats "black vote" republican roosevelt
1936 "black voters"

I hope this helps.

Thank you for the tip.
-juggler