"Portrait de Guido Sommi."

ON THE BLOCK: A seldom-seen Tamara de Lempicka painting that belonged to fashion illustrator Kenneth Paul Block and his longtime partner Morton Ribyat will go under the gavel at Sotheby’s Nov. 14.

“Portrait de Guido Sommi” will be part of the Upper East Side auction house’s Evening Sale of Impressionist & Modern Art. The striking work of Sommi, one of the artist’s former lovers, is expected to garner between $4 million and $6 million, according to Jeremiah Evarts, head of Evening Sales in New York.

Block, a famed fashion illustrator of the 20th century, served as the chief artist at Women’s Wear Daily. His directional drawings of the latest trends and the women who wore them also appeared in W magazine, and advertisements in The New York Times and other publications. He routinely traveled to Paris to cover couture shows, and his partner Ribyat, a textiles designer, often accompanied him.

A couple for more than 60 years, the two men met at a concert in New York in the Forties. During a trip to Paris in the summer of 1969, they purchased the Lempicka painting at the Galerie du Luxembourg at 2 Rue de Tournon. For most of the past 47 years, the piece of work has not been outside of the Riverside Drive apartment that Bloch and Ribyat once shared. (Block died in 2009 and Ribyat died in March of this year.) The one exception was when the duo loaned Portrait de Guido Sommi to be included in the Lempicka retrospective in Milan in 2006. Other requests to exhibit the painting were declined. “We would miss it too much if it was gone,” Block once said.

Until his retirement in 1992, Block captured the collections of American and European designers including Balenciaga, Chanel, Norrell, Halston, Galanos and Yves Saint Laurent. Ribyat’s designs were used for fabrics, curtains and wallpaper.

Evarts described Sommi as “a very fashionable Milanese man who descended from a noble Italian family,” and also was the artist’s lover. “She had a reputation for having fun with models. This particular model was one that she actually was quite infatuated with. They had a wonderful tempestuous love affair. He appears in a few of her paintings.”

It is believed Block and Ribyat knew the artist. Sotheby’s officials were surprised to find the painting, with Evarts describing it as “a real discovery.” An added bonus was their ability to spruce up “Portrait de Guido Sommi.” Evarts said, “We cleaned it and now it soars — the colors are so bright. That was a thrilling transformation to watch.”