(The ). Strictly speaking means the Czar of Muscovy; the King of Muscovy was called the White King from the white robes which he wore. The King of Poland was called the Black King.
“Sunt qui principem Moscoviae Album Regem nuncupant. Ego quidem causam diligenter quielebam, cur regis albi nomine appellaretur, cum nemo principum Moscoviae eo titulo antea [Ivan III.] esset usus ... Credo autem ut Persam nune propter rubea tegumenta capitis `Kissilpassa' (i. e. rubeum caput) vocant; ita reges Moscoviae propter alba tegumenta `Albos Reges' appellari.” —Sigismund.
“The marriage of the Czarevitch with the Princess Alex of Hesse (2 syl.), will impress the Oriental mind with the expectation that the Empress of India and the White Czar will henceforth ... labour to avoid the ... mischief of disagreement.” —The Standard, April 21st, 1894.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894