I know my history. We're talking the 60's not the 30's. In 1960 JFK had more than twice the black vote tricky Dick had. Hey it sucks for the dems more that they can't claim honest Abe as their own. Nope, dat score goes to the other side.
Black folks began moving their allegiance to the Democratic Party when they cast over seventy percent of their votes for Roosevelt in the... um... er... let me check... the 1930s.
This was repeated in 1948 when they voted in mass for Truman after he desegregated the military.
Johnson, expectedly, garnered the overwhelming majority of Black votes in 1964, having signed the Civil Rights Act legislation.
Nixon, completed the "tipping of the scale" with his Southern Strategy in the late 60s and early 70s.
What you may be referring to, but did not know how to say it, is most of the total population of Blacks began voting heavily for the Democrats in the 1960s but the reason for this was:
1. Most Blacks lived in the South...
2. Blacks in the South were not allowed to vote in mass until the 60s.