//There's no such thing as "positive discrimination". It's just discrimination.//
Exactly so, Pixie.
The principles behind the "positive" bit are these
1. We shouldn't choose whites in preference to blacks.
2. We should have an equal playing field whereby people are selected on merit.
(Everybody happy so far?)
But -
3. Every black, while the slave chains and shackles have in a literal sense been long since removed, still suffers an inheritance of disadvantage: typically more poor, with fewer opportunities for higher education, and, then, met with prejudice and denied advancement when able to overcome these obstacles.
Redressing these historic impediments requires firstly acknowledging them, and secondly compensating for them. This is what the policy of "positive discrimination" means: we take "affirmative action" to create a level playing field by adjusting for inherited inequalities.
Example:
Harvard entrance exam requires a pass mark of 80%.
A "privileged" white male candidate for admission achieves that score. An historically encumbered black entrant might only achieve 70%. But the (arithmetically) failed candidate is (allowance being made for his undeserved impediments to success) equally talented and equally worthy of a place.