I believe that the decision hinges on what serves the play. The play's the thing. If the writer has specified an ethnicity for a particular role, and the plot or content of the play hinges on that character being that ethnicity, then it should not be cast colorblind. For example, to cast a Latino in the role of Tevye in Fiddler on the Roof, a play about the Jewish experience, would not serve the play. Conversely, to cast a white man in the role of Othello would also not serve the play. It is true of course that directors develop certain artistic concepts which oppose the author's original specifications. In that case, it is a totally different play than the original which the director is presenting, and casting methods may change accordingly.
But regardless of how far you take colorblind casting, the reality is that new plays must be written which encompass the black, or the Latino, or the Asian experience. New companies must be formed which are dedicated to the honest depiction of minority cultures onstage, and to producing the writers who hail from those cultures.
It is crucial that the demographic of the actors onstage come closer to matching the demographic of the audience's community. Not only for the honest portrayal of all cultures in America, but for the sake of ticket sales. Who besides the white bourgeoisie wants to see plays about the white bourgeoisie?