The cardinal rule of wine and food pairing is that the wine must be sweeter than the food. Since even dark chocolate has a significant percentage of sugar in it, only dessert wines will match the sweetness level of chocolate. Dry wines will taste thin and tart and may make the chocolate taste excessively bitter.
The best pairing is a good semi-sweet or dark chocolate with Madeira Malvasia or Bual, such as this one. Banyuls, Grenache-based fortified wine from Southern France, is also a great pairing. Both wines have good acidity which helps give life to the viscous chocolate.
Port, Sauternes or Tokaji (at least puttonyos 3) are also good choices depending on the flavors you want to bring out, as are late-harvest Zinfandels.
But if you are tempted to pair a dry Cabernet, Merlot, Pinot Noir or Zinfandel with chocolate your date or guests will smile and go yum while thinking you’re an idiot. And don’t even think about Champagne.