520Fifteen Central Park resident Kyle Blackmon, whose sale of Sanford Weill‘s penthouse there to fertilizer oligarch Dmitry Rybolovlev for $88 million still holds the record for the most expensive apartment sale in New York, has left the Brown Harris Stevens brokerage for upstart Urban Compass. BHS is partly owned by Will and Arthur Zeckendorf, the lead developers of Fifteen, and Arthur is one of Blackmon’s neighbors in the building where he’s been one of most productive broker-specialists. You’ve gotta wonder how welcome he’ll be at the sales office for the Zeckendorfs’ latest condo tower, 520 Park (shown), which the Real Deal pointed out last week, is actually rising at 45 East 60th Street, “and is not on Park at all.” If they sell its penthouse at its $130 million asking price, the developer-brothers will likely laugh about all of this all the way to the bank.