Venezuela's economically crucial black market for dollars was all but frozen Friday following the money-laundering arrest of an owner of a small Florida financial firm, adding to tensions between the U.S. and Venezuelan President Hugo Chávez. The arrest has reverberated through the Latin American country because the firm, Rosemont Finance Corp., serves as a key U.S. clearing house for dozens of black-market brokerages -- trading houses that exploit loopholes to sell dollars despite an official Venezuelan ban on private firms buying and selling currency at unofficial rates. The federal case has ensnared millions of dollars from these trades and the...