As far as the question goes. Yes, I am fully aware of not only repression but the after effects of slavery.
In comparing migrations to the U.S. one has to keep in mind that they were not all the same even though they looked like it on the surface. The migration of African slaves into the Americas was unique in that it was the only one where the immigrants were not accommodated within the American mainstream within 100 years. Today you would not be able to tell that there was ever anti-immigrant posters in Manhattan during the 1910s and 20s. You wouldn't even be able to tell that there was ever even any real discrimination against Italians. Less than 50 years after the mass migrations of the early 20th century Italians were able to own businesses, buy houses in affluent neighbors and amass wealth.
Eugenics was not born out of a need to create a better white race, it was born out of a need for white supremacists to define 'whiteness' itself. As physiological explanations for race lost out to theories of racially defined intelligence, the definition of 'white' began to expand to the Polish, Irish, Italians etc. However even before the definition itself expanded, the migrations of 'non-white' European immigrants were relatively similar to each other. None of them possessed the characteristics of the forced African migration to America.
That is not to say Italians, Polish and the Irish were not discriminated against. It would be foolish to make such a claim. What is being asserted is that the discrimination was entirely different and less extreme. This has been attributed to the fact that most of these groups migrated through the East Coast. In contrast, most Africans in America had been sent to the South. Southern resentment over losing the Civil War was expressed in the many Southern laws barring blacks from owning businesses. It was expressed in the segregation of schools. The South however was not alone in this discrimination as even in the early 20th century blacks had yet to make a real mark in Northern politics or academia.
The point I'm trying to make is that though it may seem to some that 'discrimination of blacks' and 'discrimination of Italians' might sound the same, the historical evidence proves that they are not. It's not that one was discrimination and the other was not. It's that they were different kinds of discrimination. Whereas most European groups had been absorbed by the American mainstream less than 50 years after their arrival, blacks endured a systematic discrimination that lasted well into the second half of the 20th century.