I don't think the Libyan rebels care if the blacks are mercenaries. I think they kill them because they're black.
Don’t worry, Amnesty int’l has called on the NTC to stop such abuses. /s
Don’t read too much into the article.
The wider point is that Black Africans don’t like Arabs and vice versa. Gaddafi for all his faults was not as racist as the typical Arab, this is why he is liked through out Sub-Saharan Africa.
However, Nigeria has never liked Gaddafi. Gaddafi’s was behind the major crises in West Africa (Sierra Leone and Liberia). Nigeria expended considerable blood and treasure (lost a lot of soldiers) stabilising those two nations.