Blending (or High Gravity Brewing) is a technique that allows you to brew a larger end-batch volume without buying larger fermentation equipment. A higher specific gravity beer (strong beer)is first fermented and then blended with water to create a lower alcohol beer with less IBU, SRM, and final gravity than the originally fermented beverage.
Blending can also mean the mixing of two independently fermented beverages. This can be the same beer or different beers combined for a different flavor. Commercial examples include Newcastle (two differently fermented beers blended for a consistant flavor), Budweiser-Coors-Miller (the same beer fermented in different tanks and then blended for a consistant taste), and finally bartender-mixed blends such as Black & Tan (Guiness Stout and Bass Ale) or Half & Half (Guniess Stout and Harp Lager), which are two different beers mixed after the serving container for a differently tasting beverage.
--By WortMonger, member of HBT.com 11:40, 4 September 2008 (CDT)