Sotheby’s is preparing to bring works from the collection of the late dealer and financier Robert Mnuchin to auction this May in New York, adding a fresh trove of blue-chip material to a season the house hopes will build on its blockbuster November and a strong start to the spring sales. The consignment is made up of 24 works from the personal collection Robert assembled with his wife Adriana Mnuchin, long known among collectors for its focus on museum-quality examples of postwar abstraction and modern art. The sale will be led by Rothko’s monumental 1957 canvas *Brown and Blacks in Reds*, estimated at $70 million to $100 million, along with a second Rothko from 1949 estimated at $15 million to $20 million. Standing nearly eight feet tall, *Brown and Blacks in Reds* dates from Rothko’s most important decade, when the artist developed the luminous stacked bands of color that defined his mature work. Executed in one of the artist’s coveted red palettes, the painting once belonged to Joseph E.…