1590s, from Russian beluga, literally "great white," from belo- "white" (from PIE *bhel-o-, suffixed form of root *bhel- (1) "to shine, flash, burn," also "shining white") + augmentative suffix -uga. Originally the great white sturgeon, found in the Caspian and Black seas; later (1817) the popular name for the small white whale (Delphinapterus leucas) found in northern seas.
Proto-Indo-European root meaning "to shine, flash, burn," also "shining white" and forming words for bright colors.
It forms all or part of: beluga; Beltane; black; blancmange; blanch; blank; blanket; blaze (n.1) "bright flame, fire;" bleach; bleak; blemish; blench; blende; blend; blind; blindfold; blitzkrieg; blond; blue (adj.1); blush; conflagration; deflagration; effulgence; effulgent; flagrant; flambe; flambeau; flamboyant; flame; flamingo; flammable; Flavian; Flavius; fulgent; fulminate; inflame; inflammable; phlegm; phlegmatic; phlogiston; phlox; purblind; refulgent; riboflavin.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit bhrajate "shines;" Greek phlegein "to burn;" Latin flamma "flame," fulmen "lightning," fulgere "to shine, flash," flagrare "to burn, blaze, glow;" Old Church Slavonic belu "white;" Lithuanian balnas "pale."