Thanks! Now I just have to figure out the difference between "which" and "which one"...
'Quel' means which and 'lequel' means which one.
Using the above sentence as an example, we have its English counterpart as: 'Which horse is black?', not 'Which one horse is black?'
But, they can also often mean the same thing in English, but that is not the case with French. For example, 'Which is my book?' and 'Which one is my book?' more or less mean the same in English, but their French counterparts, that is 'Quel est mon livre?' and 'Lequel est mon livre?' do not.
Hope this helps. :)
While it may seem as if quel and lequel can both be used at this point, they do not have the same meaning. The lequel interrogatives do mean 'which one(s)' and should only by used for specification (Les livres, lesquels? or L'étudiant, lequel?). The quel group means a vague 'what' which is why more words are needed to determine the context of the meaning (ex. Quel couleur... or Quelle heure... or Quel est ton/votre [masculine object]).