When a mysterious new storefront opened last month in the heart of brownstone Brooklyn, residents buzzed and blogged about what was behind the black door.

A red print curtain blocked the view inside, and a hand-drawn sign in the window, “TAO INC,” provided the only clue.

Would it be a new restaurant or fancy boutique like others along Smith Street, smack in the middle of the piping-hot neighborhoods of Cobble Hill and Boerum Hill, Carroll Gardens and Gowanus?

Or, in a place where the median income is $113,000 and the average home price $684,000, could it be a brothel next door?

Online speculation spread about the goings-on at 257 Smith St.

Hints began to raise alarm among residents of the family-friendly and celeb-studded community, home to Ethan Hawke and Rose Byrne, and where Norah Jones spent $6.2 million on a renovated carriage house featured in “Eat Pray Love.”

One observer claimed to have spotted a young woman, dressed in only a nightgown, open the door.

“Something sketchy seems to be happening inside,” posted Katia Kelly on her Pardon Me for Asking site on Nov. 15. “It would appear that we now have a house of ill repute.”

The Post ventured beyond the black door to find that the area’s newest neighbor was, in fact, just that — a bordello barely masquerading as a massage spa.

Several ads on sex-seeking Web sites raised obvious red flags.

“Every day we have 8 girls working . . . u can pick,” Tao claimed in a listing on Backpage that showed busty Asian women in lingerie and encouraged men to “Come be a King.”

A Post reporter, posing as a massage customer, called the number listed online on Wednesday and asked for an appointment. A woman with an Asian accent answered and sent the address and massage prices — $50 for an hour, $40 for half an hour — via text. The message was embellished with 16 heart emojis.

He was ushered in by a petite 20-something Chinese woman wearing tight jeans shorts, high heels and a white lace baby-doll top.

She spoke almost no English but introduced herself as “Coco.”

Coco smiled and giggled girlishly as she took the reporter by the hand and led him down a dark, narrow hallway to a small purple room with a thin mattress on the floor. She asked for payment, and he handed over $50 for an hour massage. He was instructed to take off his clothes, skivvies included. He did.

He was told to lie face-down on the mattress, which was covered with a print sheet, red towel and white gauze fabric. She removed her heels and sat on his back. She kneaded the muscles on his back and shoulders with her hands for about 15 minutes.

Moaning sounds came from an unseen woman in a nearby room, along with a man’s voice and a clanking belt buckle.

She then had him turn over. “What you like?” she asked.

“Well . . . everything,” he replied.

The Tao ad offered hot stones and a milk and rose bath, so whatever she was proposing was unclear. She pouted and rubbed her fingers together, indicating money. He got up and fished out $160 in 20-dollar bills. “One more,” she said. He complied.

Coco proceeded to disrobe, taking off her black bra and red panties, which she twirled in front of the reporter’s face.

She left briefly and came back. Coco proceeded to disrobe, taking off her black bra and red panties, which she twirled in front of his face. She grabbed the reporter’s hand and tried to induce him to touch her breasts. He refused.

She then pulled out a condom, unwrapped it and moved to put it on the reporter. He declined again. She seemed confused that he didn’t want sex.

Coco struggled to make herself understood. “I learn English,” she said but looked puzzled when asked how old she was and how long she had been in the country. She managed to convey that she was from Hong Kong and lived in the back of the parlor.

She also said — through mispronounced words and gestures — that she must remain on the premises 24 hours a day, and wasn’t allowed to leave.

Those indicators can be a sign that a brothel is engaged in human trafficking or coercion, according to experts.

As he was leaving, the reporter spotted two other young Asian women in a front room. They were dressed in bathrobes and seated on a futon.

Cops in the 76th Precinct are aware of Tao but have taken no action, according to a police source. The landlord did not return calls seeking comment.

Sex-selling massage parlors are increasingly common in New York, according to an investigation by the Contently Foundation that ran in The Post last year. And they are making inroads into well-heeled neighborhoods like Cobble Hill. One has reportedly opened on the Upper East Side.

Experts say such places, owned primarily by independent Chinese and Korean operators, are spreading to other metro areas, suburbs and small towns across the United States.

But the national epicenter of the Asian sex trade is located in Flushing, Queens, and such brothels purportedly now outnumber Starbucks in New York, and collectively do more than $1 billion a year in business.