Translation:Du pain noir
I'm assuming you mean why isn't it 'du noir pain'. The short answer is that generally adjectives in french come after the noun, hence 'du pain noir'. It is important to note there are exceptions to this, generally remembered using the acronym "B.A.G.S":
Beauty - La belle femme (the beautiful woman)
Age - les jeunes enfants (the young children)
Goodness(/Badness) - une bonne idée (a good idea)
Size - une grande maison (a big house)
Of course, there are exceptions to THIS rule as well, but its a good rule of thumb to use.