The best-known names in asset management and investment banking are taking opposite sides in the debate over the classic way of building a portfolio — 60% stocks and 40% bonds — after a disastrous performance for the 60/40 model last year.

BlackRock says the losses — the worst in nominal terms for a 60/40 portfolio since the financial crisis of 2008-09 and the worst in real terms in a calendar year since the Great Depression — show that the structure is outdated.