(Adds Alpha comment that it is reviewing ruling in paragraph seven)
By Tom Hals
May 28 (Reuters) - Alpha Natural Resources Inc must pay for the legal defense of Donald Blankenship, who is facing a criminal trial over a coal mining disaster in 2010, when he headed Alpha's Massey Energy Co business, a Delaware judge ruled on Thursday.
Blankenship was indicted in November for his role in the explosion at Massey's Upper Big Branch mine in West Virginia in 2010, which killed 29 people. It was the worst U.S. mine disaster in four decades.
In February, Massey refused to continue paying Blankenship's defense costs, arguing he was no longer entitled to the benefit after he was indicted, according to court records.
Blankenship, who was chief executive officer from 2000 to 2010, has pleaded not guilty to violating federal safety laws. If convicted, he could face a total of 31 years in prison.
Blankenship is accused of conspiring to tip off workers about inspections so they could cover up violations. The indictment also said that after the explosion, he misled the U.S. Securities and Exchange Commission about Massey's safety practices.
Andre Bouchard, a judge on Delaware's Court of Chancery, ruled in favor of Blankenship and rejected the company's reading of its charter and other agreements. Blankenship said his unpaid fees totaled $5.8 million as of April, according to the ruling.
Alpha said in a statement it is reviewing the ruling.
Blankenship retired in December 2010 as CEO and chairman. A month later Massey agreed to be acquired by Alpha Natural Resources Inc of Bristol, Virginia, for about $7 billion. Alpha has not been accused of wrongdoing.
Massey and Alpha are both incorporated in Delaware. (Editing by Richard Chang and Jeffrey Benkoe)