Translation:Du pain noir
I think it's because 'du' means 'some of', you can have some of the bread, but you can't have some of the boy or some of the robe
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.
Okay, thanks a lot that's very helpful.
For the BAGS exceptions, for instance, for goodness/badness, does the exception apply to all qualities of the possible adjectives, like good, alright, horrible, awful? And what about if its something like 'very bad' or 'really good'?