The lineup was like the annual gathering of of Harlequin Romance or Mills & Boon cover stars, with models dressed in saucy getups and slinking their way down the runway of a bar called Apollo's Muse ...

With its $31 billion iShares U.S. Equity Factor Rotation Active exchange-traded fund, BlackRock has a major hit on its hands. But the fund’s biggest test may be yet to come.