Several thousand pilgrims and tourists, packed at St Peter's Square on Tuesday to stare at the slender stovepipe jutting from the chapel's brown tiled rooftop, gasped as the smoke appeared just before noon (1000 GMT).
The 115 voting cardinals sequestered in the chapel were to break for lunch and reconvene in the afternoon for the day's final session of secret balloting.
White smoke - and the pealing of bells shortly afterwards - eventually will tell the world that the church's 265th pontiff has been chosen to succeed John Paul II, who died on 2 April at age 84.
It was the second time since Monday evening's first ballot that the smoke confused onlookers.
The crowd was quiet as it tried to determine the colour of the smoke, which began grey. As the plume darkened, the pilgrims began to disperse as the chimney stopped spewing the black smoke.
But when the bells of St Peter's Basilica tolled as they do each noontime, many still in the square thought it was the signal that a new pope had been elected.
Watchers wait to see the white
smoke from the Sistine Chapel
Only when the chimney spewed more black smoke was the outcome of the morning vote clear to the crowd.
Murmurs in several languages – "nero", "black" and "schwartz" - then swept the square.
Tuesday's inconclusive two morning ballots followed an early mass in the cardinals' high-security Vatican hotel.
The prelates from six continents and 52 countries were to return to the chapel at 4pm for two afternoon ballots, with a new plume of smoke expected by 7pm.
If the afternoon session also fails to produce a pope, the conclave will resume on Wednesday morning.