Unlike milk chocolate, bittersweet and unsweetened chocolate bars are both quite dark -- almost closer to black than brown. The difference in hue is due to the cocoa content. Confectioners use very precise definitions to distinguish between chocolate types. Unsweetened chocolate, also known as chocolate liquor, is cocoa nibs ground into a smooth liquid, then solidified into bars with no additives. Bittersweet chocolate is at least 35 percent chocolate liquor and no more than 12 percent milk solids. Bittersweet and semisweet chocolate fall under the same umbrella, but bittersweet chocolate typically has at least 50 percent chocolate liquor.

A 1-ounce square of unsweetened chocolate has 145 calories, 15 grams of fat, almost 5 grams of dietary fiber and 0.2 gram of sugar. Although the nutritional profile isn't bad, unsweetened chocolate is unpalatable in plain form. It is used almost exclusively in baked goods and candy. The same serving of bittersweet chocolate with a 45 to 59 percent cocoa solids content has 155 calories, approximately 9 grams of fat, 2 grams of fiber and 13.5 grams of sugar.