There is a long history of American Jewish support for black causes. The 2nd, 3rd, and 4th national directors of the NAACP were Jewish. The man who provided most of the original funding for the NAACP was Jewish. During the Civil Rights movement, American Jewish publications prided themselves on the role played by Jews.
Then came the Crown Heights race riot of 1991, in which mobs of black thugs attacked Jewish people at random. It seems like it has all been downhill since then. According to the New York Times, the income gap between Jewish Americans and the general population is greater than the income gap between whites and blacks. The people at the bottom tend to view the people at the top with great jealously. Then there is the Israel/Palestinian conflict in which blacks increasingly sympathize with the Palestinians.
The breaking of the traditional black-Jewish alliance could spell disaster for the Democrat Party. Blacks make up the largest voting block. However, Jews provide funding and fill leadership roles for the Democratic Party is great disproportion to their overall numbers.