PITTSBURGH — The investment group led by Chicago billionaire Neil Bluhm has increased its stake in Pittsburgh’s Rivers Casino, three months after a principal owner testified in a real estate tax appeal that the casino has lost equity and was a “terrible investment.”

Holdings Acquisition on Wednesday received approval from the Pennsylvania Gaming Control Board to buy the 20 percent stake held by other investors once led by the late Detroit businessman Don Barden, according to a report.

The gambling board didn’t reveal the transaction price to raise Holdings Acquisition’s stake in the casino from 69.5 percent to 89.5 percent.

The move to buy a bigger stake in the casino comes three months after a principal in the investment group testified at a March hearing seeking to lower the casino’s property tax value that the casino was a “terrible investment.”

Allegheny County Judge R. Stanton Wettick was asked to decide whether to uphold the county’s assessment the casino had a taxable value of $199.5 million in 2010. The casino is seeking to have the value of the casino lowered to $117 million for this year. An attorney for the Pittsburgh Public Schools argues that the casino should be considered worth even more, $278.2 million.

Wettick has yet to rule, but in March, Ira Schulman, a principal with the casino’s majority ownership group, testified casino revenues have fallen short of projections and said the majority investors’ group almost filed bankruptcy and has lost $200 million in equity.