You’ve heard all about Long Island merlot, but what about the Malbec? Merlot might rule this over-grown sand bar, but Malbec, the large, easy-to-ripen black grape best known as Argentina’s premier variety, is showing surprising potential in Long Island’s maritime environment. It has been grown, and used, on Long Island for quite some time—usually ending up as a minor component in Bordeaux-style red blends. But now, as Long Island vintners continue to experiment with grapes and techniques, varietal Malbec bottlings are becoming available. Macari Vineyards in Mattituck released its 2005 Malbec ($22) a couple months ago and it’s impressive. It’s…