Kei Okumura is an unrepentant chocoholic with a preference for the dark side.

The Los Angeles mother of an infant son reduced her chocolate munching during her recent pregnancy, but now she's back at Trader Joe's several times a week, purchasing bars of dark chocolate.

"I always have two to four bars of different chocolate to nibble on daily," said Okumura, who figures she spends about $12 a week on the candy and has spent up to $8 on a single bar.

At a time when overall chocolate sales are as flat as a Hershey bar, customers such as Okumura are fueling a surge in consumption of dark chocolate, typically characterized by its bitterness and hints of coffee and berries. And it's part of a big increase in sales of premium chocolates as well.

Hurt by obesity and sugar concerns, U.S chocolate sales fell 1 percent last year to just under $16 billion, according to Packaged Facts, a market research company.

Yet the dark chocolate segment of the market grew almost 15 percent to $4 billion last year. It now accounts for 25.1 percent of all chocolate sold and is expected to gain an even larger share in coming years as consumer tastes shift. Milk chocolate sales slid 5.5 percent to $11.7 billion.

Okumura, 35, said she was intrigued by the claims that dark chocolate is more healthful than other forms of the confection, but she really likes the taste.

Dark chocolate has gained cachet as a food — such as almonds, blueberries and red wine — that studies say is good for the heart.

That's because it has a high concentration of plant compounds called antioxidants, or flavonoids, which a series of small studies have shown to improve cardiovascular function, said Dr. Leslie Cho, a cardiologist and researcher at the Cleveland Clinic, a large research hospital in Cleveland.

"These are intriguing studies but they all need more research," Cho said.

Physicians once thought that large doses of vitamins E and C, also both antioxidants, would improve cardiovascular health, but new research has found that large doses of the vitamins had no benefits and in some cases were actually harmful, Cho said.

Regardless, consumers are jumping at the chance to redefine chocolate as a health food, said Curtis Vreeland, a Packaged Facts senior analyst.

As a result, the number of new dark chocolate products tripled to 926 last year from 2002 and now account for 63 percent of all new chocolate products introduced in 2006, Vreeland said.

A year ago, the Burlingame, Calif.-based Guittard Chocolate Co. introduced Nocturne, a bar that has 91 percent cacao content. That means 91 percent of the confection is either ground-up cacao bean or pure cocoa butter and it is pretty bitter, said Gary Guittard, president of his family-owned business.

"We thought the percentage would be too high for most people," Guittard said. "This was basically a product for chefs, but we were surprised how it took off with consumers. People will buy it and eat a square a day."

Mainlining cacao, however, isn't for everyone.

"I like the sweetness and creaminess of milk chocolate," said Natalie Mach, a Thousand Oaks, Calif., nurse who also has solid chocoholic credentials.

Mach 46, trolls Bristol Farms after Valentine's Day and Easter, snapping up the fine chocolate brands in holiday-themed packaging at half price. Mach also likes to frequent See's Candy and Godiva chocolate shops.

Mach is a typical consumer of the booming premium chocolate business.

Sales of premium chocolate — anything that sells for $8 a pound or more — jumped 10.9 percent to $2.7 billion last year, Vreeland said. See's Candy starts at about $14.50 a pound and Godiva sells for $38 to $40, depending on what's ordered.

People are searching for chocolate, both milk and dark, with better ingredients, Vreeland said. They are buying chocolate where the cocoa comes from a single county or farm, is infused with exotic flavors such as lavender or cranberry, or is organic.

Okumura has even sampled a Vosges Haut-Chocolat bar that's flavored with ginger, wasabi and black sesame seeds. Vosges sells it and other exotica such as a bar made from Indian green cardamom, organic California walnuts, dried plums and Venezuelan dark chocolate for $7 for 3 ounces.

"It is very funky, but I always try out the new chocolates," Okumura said.