Thanks to Bill Fregosi for the historical background on Harolyn Blackwell's
Met success in the early '90s. One point that might be overlooked is that
Pavarotti has very often requested familiar, well-liked colleages to appear
opposite him, and very often the Met and other opera houses will go along
with his casting suggestions. (Another example of a soprano colleague
Pavarotti apparently got along with very well is June Anderson.)
Of course Pavarotti is not alone in this; other superstars have their
favorites as well.
None of this, of course, means that Ms. Blackwell is anyone's particular
cup of tea in the role of Oscar, but it's not a matter of "affirmative
action" so much as "who you know."