I've been doing a lot of research on this for a school project on this movie. Undoubtedly, it was a major factor for the second rise of the KKK. All of a sudden, the KKK was portrayed to be heroic, protected America from the "ruthless" black savages who want to take over the government and pass the dreaded "interracial marriage laws" and other terrible laws. One scene with the primarily black government shows them debating over a law (With their feet on the table eating fried chicken and drinking beer) that requires white men to salute black men passing on the streets. Many white southerners were scared at the threat black Americans posed, and the KKK seemed like a good solution to this problem. The movie was released in 1915, and after amazing reception, by 1920, the KKK had around 4.5 million members. It also caused many lynchings. A cause for this was the extreme patriotism associated with the movie, and thus with its racism. This is enforced with the very last statement of the movie, "Liberty and Union, one and inseparable, now and forever".

Also, the actors were not black. They were white with black makeup on.

Ive also done a schoolproject on this. Keep in mind during the release of this film the civil war wasnt that far away and memories of the ruined south were still very much alive. Slavery which made the south (quite) rich and which was deeply ingrained in society was destroyed along with their landscape, I recall sources citing the south was reduced to a new frontier. A lot of symbolism can be found in the film with the rape of the fair southern maiden by northern brutes. That the film would receive positive feedback in the south was to be expected with the kkk portrayed as the only thing standing between order and total anarchy in the south during the reconstruction. In the north another problem arose which was the influx of afro Americans looking for jobs in the industrial cities. Hope this somewhat answers your question can give sources/correct spelling when im home from work.