Wine Blending: Why and How
Wine blending is best done just before bulk aging to allow the separate wines to "make friends" with each other prior to the aging process.  Blending is one of the things that makes French Bordeaux wines so good.

Blending Links

There are two main things to know about blending: What it can do and what it can't.

What it can do is help balance flavors, acid and tannin levels. Blending two or more good wines can make an excellent wine.
What it can not do is make a good blend by blending a bad wine with a good one.  I have read this in several sources and I had to prove it to myself by trying to blend a bad wine with a good one.  The blend was no better than the bad wine, so I put a vinegar culture in the bad wine.  The bad wine eventually made a very good vinegar.

Another tidbit: blending an aged wine with a young wine typically results in a young blend.  The subtleties of the aged wine are overwhelmed by the roughness of the new wine.  The solera aging system developed by the Spanish and Portuguese (called fractional blending) is the only successful age blending system I know of.  This system is not suited to home wine making due to the space, time and capital requirements.  The best implementation of this scheme is used to make port and requires eight barrels.  The oldest barrel is 8 years old, the next is 7 years old and so forth.  A simple example of this follows where the first number is the percent removed and the second is the age of the barrel: 25% 8, 21.4% 7, 17.9% 6, 14.3% 5, 10.7% 4, 7.1% 3, 3.6% 2, 0.0% 1.  With this scheme, 78.6% of the blended wine is aged 5 years or more.  The barrels are then refilled with wine from the next oldest barrel, so the 8 year old barrel is topped off with 7 year old wine, the 7 year old barrel is topped off from the 6 year old wine an so on.  Note that the youngest wine in the blend is 2 years old and that all of the barrels contain a mixture of different ages.  Since only 25% of the 8 year old wine is removed, this barrel will contain wine that is much older.  For more information about the solera aging method see The Wine Wizard column in the Winter 2000 issue of WineMaker magazine (back issues available from http://www.winemakermag.com).

In my opinion, the goal of blending is to produce the perfect balance between all of the flavors present in wine at the peak of the aging period.  Please see "How Wine Ages" at the link above to see how key  flavors change over time.   Examples of a good blends are Bordeaux, Meritage (pronounced like heritage, its an English word, not French) and Claret (pronounce the "t" this is another English word, not French)  wines.  These wines are very similar and are typically a blend of Cabernet Sauvignon, Cabernet Franc and Merlot.  Cabernet Sauvignon is a strong full bodied wine, Cabernet Franc is a softer bodied wine and Merlot is a light bodied wine.  All three varietals contain black current flavors.  In addition, Merlot contains plum flavors.  Cabernet Franc brings raspberry flavor into play.  Sangiovese can also be used in a blend like these to bring cherry flavor to the final wine.   The key to blending is your ability to distinguish the different flavors and judge their intensity.  Based on your tasting impressions, you create several different blends (ratios) of these three wines.  Next, you and a friend or two taste the blends, spit them out and make notes of your tasting impressions. You spit the wines out because you will be tasting 9-15 different blends and the amount of alcohol contained in 9-15 "tastes" will render your judgment useless.  Select the top three blends by voting and make enough of these top three blends for two follow up tastings 2 and 3 days later.  The flavor of the blends will change after 2 days in the bottle, the effects of any alcohol you "inadvertently" swallowed will have worn off and you will have two more chances to confirm your one best blend.

Blending Links:
 Winemaker magazine blending article
 Winemakers Emporium
 VinExpert
 The Wine Merchant