This old political poster was created by a political party called the Herstigte Nasionale Party (Reestablished National Party). The HNP is one of the minority parties who still believe in racial segregation. Essentially, that's more or less what this poster tries to convey - Bly Blank My Volk means Stay White My People and tries to convey the message that the Afrikaner people should remain segregated; white; and Afrikaans-speaking.
Essentially, their intent is to protect the interests of the "old" Afrikaner nation. I think that their fear is that because Africa is a continent with a huge black majority that their identity as a people is at risk... which I guess is a fair concern.
Whether or not you and I subscribe to the ideals of the HNP isn't really relevant to me - what I find it interesting that a people feel the need to protect their identity with such fervour. It's not really something that I'm able to identify with.