It's a Stewart sett with a black field. The "Royal" is the same sett with a red background, and the "dress" has a white background.

I don't know if it's related, but there is a story about the "Black Stewart". Maybe, in addition to changing his name, he changed the field of his tartan as well.

"The Stuarts of Bute are descended from John Stewart, a natural son of Robert II, who was granted the lands of Bute, Arran and Cumbrae by his father. He was known as the 'Black Stewart,' to distinguish him from his brother, John of Dundonald, who became known as the 'Red Stewart.'

The grant of lands on Bute was confirmed in 1400 by a Charter from Robert III, along with his becoming Hereditary Sheriff of Bute. Following the reign of Mary, Queen of Scots, the family adopted the alternative spelling. Sir James Stuart of Bute became a Baronet of Nova Scotia in 1627. He supported Charles I, who appointed him Royal Lieutenant of the West of Scotland, but following the King's execution, fled to Ireland. His estates were confiscated and he was forced to pay a substantial sum of money to have them returned to his family.

The Stuarts of Bute supported William of Orange and Queen Mary, and in the reign of Queen Anne, Sir James' grandson, another Sir James, was one of the Commissioners for the negotiation of the Treaty of Union between Scotland and England. In 1703, he was created Earl of Bute.

During the 1715 Jacobite Uprising, the Stuarts of Bute supported the Government. John, 3rd Earl of Bute was a close friend of George III and was appointed First Lord of the Treasury. In 1763, he concluded the Treaty with France which brought the Seven Years' War to an end. John, 4th Earl of Bute was created Marquess of Bute in 1796."