Some were getting refugee status, such as the Sudanese, since they were getting slaughtered. I think the Congonese get it also, but they are rare, since the travel is so far. Most from from Eriteria, Chad, Ethopia and Kenya, now which are war zones.
It doesn't have to do so much with African origins (although that does have some effect), but rather a the illegal immigration issues.
Look a country as big as ours can sustain Illegal immigrants, imagine what it does to a country the size of Israel!
I can totally understand that, if they let the problem overwhelm them Israel can become an African majority country in a matter of decades.
There is no threat of Africans becoming a majority or even out of the single digits in percents, rather it has to deal with every problem western countries (like the US) face when dealing with illegal immigrations:
(1) Crime: Illegal immigrants are typically poor, uneducated and come from bad circumstances that shape their morals. Whether one wants to face it or not, crime is higher amongst illegal immigrants, whether they be white, black, yellow, brown or green! (2) They take jobs: The common misconception is they take the shit jobs no one what like picking fruit, however, in the US only 3% of illegals work in the argicultural sector. Many work in high paying construction and manufacturing!
(3) Abuse the Social and Welfare System: Israel has free healthcare and education (come of the best in the world) and a very social programs. Poor illegal immigrants abuse these systems and stretch them to the limit.
(4) Stretch out Charities: Charities provide to them like they would natives.
Some illegals are beneficials, but as a whole they utterly hurt society and drain resources! It has nothing to do with them being black (Muslim would be a different story). I personally hope (and I hope many Israelis do also), that South Sudan becomes a flower in the middle of the desert. They desire it and have the resources to become a successful nation!