The best way to do this is to define a raven as:

" A large black passerine bird (Corvus corax), similar to the crow, but larger. It is native of the northern part of Europe, Asia and America, and is noted for its sagacity.".
That is, if we take Webster 1913 as our ulimate authority. And who among us does not?.

Now, if it's not black, it's not a raven. By definition. QED