(Adds shareholder detail, BlackRock's Italian investments)
MILAN, March 21 (Reuters) - BlackRock has acquired 5.748 percent of Banca Monte dei Paschi di Siena, emerging as one of the likely mystery buyers of a stake sold by the Italian lender's biggest investor.
The holding, shown in a regulatory filing on Friday, makes the U.S. money manager the second largest investor in the Tuscan bank which is preparing for a 3 billion-euro ($4.1 billion)capital increase.
In the past month, BlackRock became top investor in UniCredit, Italy's biggest bank by assets, and second in Intesa Sanpaolo, the country's largest retail bank.
Monte Paschi plans the capital hike to help pay back a 4.1 billion euro state bailout it received last year after being hit by a European sovereign debt crisis and a derivatives scandal.
Its main shareholder - the politically connected, not-for-profit Monte Paschi Foundation - sold 12 percent of the bank this week, halving its own stake and making the lender an easier takeover target.
According to the filing by market watchdog Consob, BlackRock's stake exceeded the 5 percent threshold on March 18, the same day the Monte dei Paschi foundation sold.
The centre-left mayor of Monte Paschi's hometown Siena, said the day after the sale that the foundation's chairwoman had worked a "miracle".
($1 = 0.7256 Euros) (Reporting by Isla Binnie; Editing by Elaine Hardcastle)