The BlackRock Mortgage Investors Fund will start financing prime "jumbo" mortgage loans customized for an improved type of residential mortgage-backed securities. BlackRock executives said the old model of RMBS frustrates investors because of potential conflicts of interest and sloppy loan documentation. The new fund addresses those weaknesses. "We're really designing this to alleviate many of those concerns," said Randy Robertson, a managing director and co-head of securitized products at BlackRock. "We hope to achieve the bellwether nonagency prime securities, ultimately."