4 Things
1) You are allowed to identify yourself on the census...I think it says something like Non-Hispanic Black, then covers more details like African, African American, etc. If you tally in the confusing Afro Latino poulation is a handful millions to add.
2) Since 1973 there have been somewhere around 17 million black abortions. That's 2 generations worth of people that would have produced potentially twice to triple those aborted numbers. After subtracting out black deaths from that group and offspring since 1973, you're talking hypothetically an additional 20-25 million people that could part of the current 47 million
3) Don't disregard the waves of immigrants that came from Europe in in 1910s and 1920s. 10s of millions came through Ellis Island. Their offspring and grandkids intermingled a lot, until Roe vs Wade (1973) also begin to eat at their numbers.
4) Arabs are thrown in the white category. Hispanics/Latinos are changing again the definition of "white"