Blanc de Blancs, literally "white from white" refers to a Champagne or other sparkling wine that is made with only white grapes. For Champagne that white grape is exclusively Chardonnay. Blanc de Blancs also pair beautifully with many types of seafood, from grouper to tilapia.
As Chardonnay grown in the Champagne region has such naturally high acidity, the common thinking is that a Blanc de Blancs Champagne ages better than most other Champagnes, although its aging potential depends on numerous factors, including vine age, depth of fruit and cellar practices.
The term Blanc de Blancs is also used for other sparkling wines; the meaning is the same - only white varieties are allowed.