The flavor of the week is Tannat. As the name suggests, this grape is an especially tannic varietal that makes a dense bruiser of a wine. Nonexistent to obscure in most wine regions, it's the main thing going in a widely dispersed few. In far-flung Madiran, in the foothills of the French Pyrenées, wine must contain 40 percent to 60 percent Tannat. Preeminent producers such as the Alain Brumont's Château Montus have put Madiran on the map.

Since the wine takes a donkey's age to mellow out, this is the region that invented "micro-oxygenation," a bit of technical trickery that speeds up the aging process. In the 1880s, Basque settlers brought Tannat to Uruguay, where it has since become the South American nation's signature grape, like Malbec is for Argentina. Vineyards are grown here and there amid the gently rolling green rangeland, where the climate is tempered by the maritime influence of the Rio de la Plata. Wineries such as the rustic, stucco-walled Establecimiento Juanicó produce a mellow, agreeable version of Tannat more successfully than, say, Cabernet Sauvignon. So when I saw a sign here in Sonoma Valley that said "We Have Tannat," I was lured in for a taste.

I'd been passing by Blackstone's Kenwood tasting room for years, assuming it was not of much interest. It was conceived as a "negociant"—the industry's new pet term for bulk wine brand—owned by Constellation Brands, and widely available in the "fighting varietal" price range. Well, Blackstone's satellite Sonoma County production facility is tucked in back and pretty much left to do its own thing, producing a variety of ultrapremium-appellation and single-vineyard Sonoma County wines, most sold at the winery only. Learn something new every day.

Even if you're taking notes and asking über-geek questions, winetasting solo can be tough, especially when the bar is elbow to elbow with couples. The host, who was coincidentally explaining the story of Tannat just at that moment, shunted me on to the bar out back, a blessing. Open on weekends, the patio tasting shack is funky, dressed up just with a tabletop fountain and some piped-in wine country radio. Out of the crowd, this is where the cool folks hang out.

Blackstone Sonoma is big on the reds and unusual varietals like Dolcetto and Teroldego. In the Prestige Appellation Series, the 2003 Lake County Malbec ($20) had an aroma of, according to my notes, industrial cherry fruit—chemical but enticing, with exceptionally supple tannins. Jam fans will love the 2004 Sonoma County Zinfandel ($18), which the backyard bartender says is the jammiest Zin you may currently encounter. "I will argue that," she declares. I tend to agree, further noting it has the particular nostalgic characteristic of Knotts Berry single-serving raspberry jam packets.

The 2004 Russian River Valley Tannat ($20) was indeed a dense wine; in fact, they add a little Cabernet Sauvignon to mellow it out. It was, um, pretty good. Dark fruit, plums . . . But what have we really learned, what is the flavor of the week? Blackstone, an unexpected surprise for unique local wines.

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