Blended red wines are all the rage with wine consumers, or so some retailers would have us believe.

But curiously, the growth of this category of wine may well relate less to wine quality than to how wine is marketed and the competition for shelf space.

The explanation for this is slightly arcane. Let’s look at the retail marketing of wine:

Starting in 1978, the number of wholesalers in the United States began to shrink, from thousands nationally to a few giant companies. As some wholesalers grew in size, so did their sales portfolios — what people in the industry call “the book.”

And as books expanded to hundreds and then thousands of wines, the vast number of items any one winery made became less and less practical to manage.

Each item is called a SKU, or stock-keeping unit. A winery might have two chardonnays, three cabernet sauvignons, two merlots, a sauvignon blanc and perhaps two or three other wines. Accounting for multiple vintages that might be available at the same time, the wholesaler had to keep track of perhaps 20 or 30 SKUs from each winery it represented — not to mention warehouse it all.

This led to confusion. A retailer could ask for a case of the 2010 merlot and end up getting the 2009. No one liked the situation.

So to simplify their lives, large wholesalers began to ask wineries to reduce the number of SKUs in their portfolios. As a result, some superb and perfectly profitable wines (such as chenin blanc, sangiovese, cabernet franc, and other lesser-known varieties) were abandoned. (A few continued to be made in tiny amounts, to be sold at a winery’s tasting room or via the mail only.)

As the number of SKUs came under fire, wineries began to look at how to consolidate their lines of wines. One way was to make blends. Instead of making small volumes of a merlot, a Malbec, a petit verdot, and a cab franc, some wineries blended them all. The result was a wine they could call meritage, exclusively made from Bordeaux-type grapes.

Then came the Syrah debacle. Starting in the early 1990s, California growers began to plant syrah widely. It was known to grow well in a wide variety of soils and climates and made a pretty nice, dark red wine.

It wasn’t long before it became clear that syrah was not the gold mine some people thought it would be. True, it did make a nice red wine. But the grape made nice wine in many regions and all with about the same style, which meant that $20 versions and $10 versions were roughly the same quality. So many people traded down.

Result: There was a lot of syrah on the market in bottles that were quickly seen as overpriced. And there was a lot of syrah on the bulk market that was available for people to use to make blended reds.

This worked well with the strategy of reducing the number of SKUs. Instead of making 3,000 cases of a syrah that was hard to sell, a winery could make 1,000 of the syrah, consolidate other grapes and make a blended red.

As the varietal SKUs have been reduced, many have been replaced by blended reds that use syrah as major elements. Other grapes used widely are zinfandel, malbec, tempranillo, and sangiovese. So what do these wines smell and taste like? For the most part, they are not radically different from one another.

This must bother consumers who can’t tell the difference from one to another, right? Well, not really.

I’ve spoken with retail wine shop owners for the last few years about this. They admit that many consumers are happy to move away from mid-priced varietals toward slightly lower-priced blends.

But a number of shop owners said few people ask what grapes are in such blends. “If it tastes pretty good and the price is right, the wine sells,” said one Santa Rosa wine shop owner.

Sonoma County resident Dan Berger publishes “Vintage Experiences,” a weekly wine newsletter. Write to him at winenut@gmail.com.