Blackett was also involved in coal and lead mining, having "by the product of his mines and collieries acquired a very great fortune". He invested heavily in the local coalfield, and once spent £20,000 in an unsuccessful attempt to drain a flooded pit. He is believed to have extended his fortune by buying land in the 1660s and 1670s. He also acted as business adviser to Charles Howard, 1st Earl of Carlisle. From 1669 until his death, he was sub-farmer of coal duties. He became Deputy Lieutenant of the county in 1670. In 1672, he was involved in a dispute with the local customs officials as member of the syndicate which leased the coal export duties from Lord Townshend for £3,200 a year. He was J.P. for Northumberland from 1673 until his death.