From John Smith's 'Prehistoric Man in Ayrshire' (1895):
There is a granite boulder on Blairstone on which is cut an incised figure, popularly believed to represent the sword of Wallace; but a glance at it shows that it has not been made to represent a sword, but a cross, which measures 3 feet 6 inches long, and 14 inches over the arms of the cross, which, as well as the top of it, widen out a bit at the ends. The tradition of the neighbourhood is to the effect that Wallace laid his sword on the granite boulder, and some kind artist chalked off the outline, and cut out its representation on the stone; but it is far too small for Wallace's sword, even if it were the proper shape. For its better preservation, the stone has been surrounded by a stone-and-lime wall.
Or another theory:
Abercrummie says: "There is also upon the descent of Broun-Carrick-Hill, near to the mains of Blairstoune, a big whinstone, upon which there is the dull figure of a cross, which is alleged to have been done by some venerable churchman, who did mediat a peace twixt the King of the Picts and the Scots; and to give the more authority to his proposals, did in their sight, by laying a cross uon the stone, imprint that figure thereon." Such was, apparently, the tradition when Abercrummie wrote. It has also been attributed to Wallace as well as Bruce. The stone, which may at one time have been standing, lies apparently in the same position as it did in Abercrummie's time.
Abercrummie was the Rev. Abercrummie of Maybole. He wrote 'A description of Carrick' in the 1680s. Here he's being quoted by James Paterson in 'History of the County of Ayr' (1852).