Canon Background
https://awoiaf.westeros.org/index.php/Blackshield

In-Campaign
During a change encounter near Highgarden’s stables in 285 AC, Matthew approached Teddy Jasper and Calaila after recognizing the Jasper sigil from the Battle of the Trident. He questioned if Ser Darron Brewlan was indeed serving House Jasper and warned that the man was a thief. Matthew went so far as to suggest that Darron should be exiled to The Wall for stealing House Mullendore’s crown jewels years earlier.

In August of 286 AC Ser Matthew and some companions from The Reach happened across Ser Darron in the alleys of Gulltown. They took him unawares, overpowering and capturing the older knight. Matthew stripped Darron of his Valyrian steel and chained him in the hull of a ship bound for the Reach so that Darron could face justice for his alleged crime. However, en route to the Reach Darron was deposited at Ghaston Grey for reasons unknown to him.

In the fields south of Goldengrove Ser Matthew was out riding with his travel companion Ser Gwayne when the were set upon by a band from House Jasper. Seeing that they were outnumbered the pair tried to flee, but the swift Ser Darron was able to close in on them despite a sizable head start. The pair attempted to split apart and escape, before drawing war lances to fight off the aggressors. The matter seemed resolved when Ser Darron agreed to call off his attack in return for Blackshield’s sword, which the knight easily parted with. The men hurried north as the cold winter wind began to blow.