The 18,000-square-foot mansion at 113-115 East 70th Street that used to belong to former Coach creative director Reed Krakoff sold last February in an off-market deal for $51 million, and made it to number six on our recent list of the most expensive homes ever sold in New York City, but at the time we still didn't know who the buyer is. Now, that mystery buyer has been revealed as Related Companies CEO Jeff Blau, who put his Fifth Avenue co-op on the market for $43 million about a month before the sale and probably didn't count on it staying on the market for this long. (It's currently asking $34.5 million.) Blau had only been in that apartment for a couple of years, upgrading from a duplex in The Chatham, which he unloaded for $14 million in 2012. While both those apartments were highly customized, it would be a shame if he did too much to his new place, which Krakoff and his wife, interior designer Delphine Krakoff, renovated to exacting standards after a 2006 fire.