"At least 19 people have been killed and 25 injured. All of them were Shiite pilgrims," he told AFP, adding most of those killed were burnt to death.
"The bomb was planted in a car. The condition of some of the injured is critical," Baluch said.
Some 180 Shiite pilgrims were on their way to Iran in the buses when the bomb ripped through one of them, said Akbar Hussain Durrani, a senior government official in Quetta, the capital of Baluchistan province.
One bus carrying some 45 pilgrims was badly damaged, he said.
Mastung is some 30 kilometres (18 miles) south of Quetta.
There was no immediate claim of responsibility for the attack.
The province has become an increasing flashpoint for sectarian violence between Pakistan's majority Sunni Muslims and minority Shiites, who account for around a fifth of the country's 180 million people.
Baluchistan is also rife with Islamist militancy and with a regional insurgency which began in 2004. The insurgents demand political autonomy and a greater share of profits from the region's oil and gas resources.