Depends on how the German part looks. I mean, German is a nationality, not an ethnicity. Germany also has some black citizens. And many citizens with east Asian, Middle-Eastern, South-European and East European heritage.
As for the "native population" in Germany, the blond and blue-eyed people are actually a minority. Typical hair colors in Germany vary widely. We have redheads, blonds, brown haired people are the majority and some folks have black hair. Skin colors vary from pale to tanned. Common eye colors are blue, gray, green, brown and any mixture of these.
And the other part of the equation is, what do you consider "black"? In the USA it seems to me that anyone who has got any black ancestors is considered black, even if they are actually mixed-ethnicity and a lot paler than the average African. Mariah Carey or Collin Powell are famous examples.
And I think that is what the baby of one of the rare blond, blue-eyed German and an average equatorial African person would look like: Black hair, light brown skin and a mixture of typical native African and typical native European facial features.
Look at this photo of Heidi Klum (blond, German ex-model and entertainer), her husband Seal (black, British/Nigerian singer) and their kids. http://funadvice.com/r/bh4iibua9jr
It would look like a mix of the parents and whatever their traits were.
They would be classified as coloured in my country.
Mostly black, as it is the more dominant gene.