When Merrill Lynch decided this month to sell its asset-management operation to BlackRock -- a money manager serving wealthy investors and institutions and best known for its conservative focus on bonds and risk-management products -- analysts and investors cheered bidding up the shares of both companies. By acquiring Merrill’s $539 billion mutual fund family BlackRock will quickly broaden its stock-fund offerings and its appeal to retail customers. Merrill by acquiring just under 50% of BlackRock will do well if BlackRock can strengthen the performance and appeal of Merrill’s disappointing fund operation. But the cheering involves some wishful thinking as well. Merrill hopes to finesse its way out of its disappointing decision to embrace one of the late-1990s hottest fads: the financial supermarket. And BlackRock has to hope that investors don’t come to see it as a de-facto Merrill subsidiary inheriting Merrill’s problems without adding value of its own to the asset-management business.