BlackRock, a leading US bond investor, says banks will have to take their share of losses on distressed mortgages before it resumes large-scale purchases of new “private label” mortgage bonds, which are sold without government backing. The position taken by Curtis Arledge, chief investment officer for fixed income at BlackRock, who oversees $580bn of investments, marks the latest development in an ongoing tussle over who should bear the costs of the US mortgage meltdown. The return of private investors to the US mortgage market, now mostly financed through government-backed agencies, could have a big effect on mortgage rates and the speed of the housing recovery. Efforts to restore confidence among investors have so far failed.