In a TYPE-MOON Ace 7 - 10 year Anniversary interview, Nasu Kinoko, Takeuchi Takashi, Urobuchi Gen were asked a number of questions. while i was looking up for any interview about Alexander's name choice for another answer here i came acorss this (may have to expand Reference 8)
Q: Having the "black" one as the main character and the "white" one as the villain is a refreshing idea. Then, how about the red Saber (Saber Extra), who appeared in Fate/Extra?
Uro: That character is completely disconnected from the other Sabers. Personally, she looked like a female Gilgamesh to me.
Nasu: If I had written Fate/Zero, then Iskandar would have been a character like Red Saber. In my mind, I can only see Iskandar as a worthy opponent to Gilgamesh, but the Iskandar Urobuchi created was an excellent, wild, and unprecedented character that I could have never thought up. But I want to surpass him somehow, so maybe Red Saber was born out of my drive to create a different tyrant from Iskandar.
Just from the comments made by Nasu and Urobuchi i would suspect that when the question talks about "black" and "white" they mean the alternate forms of Saber-Arturia, Saber-Alter (black) and Saber-Lily (white) however i don't know any Fate/ work which has Saber-Alter as the main character and Saber-Lily as the villain.
Also i am aware of Saber of "Black" however because Nasu's comment i believe is talking about Saber-Nero given her reputation rather than Saber of "Red" i'd assume he's not talking about Fate/Apocrypha characters
I'm wondering who these "black" and "white" characters are and where they appear in the roles mentioned in the question?
Reference: Left is Saber-Alter, the right is Saber-Lily