Bravo retired hurt on 23 on the fourth morning of the Test, having just driven Jasprit Bumrah to the cover boundary in the fourth over of the day. He had taken a blow to the helmet in the last over of day three, off a bouncer from Bumrah. The ball had struck the right side of his helmet, and sent his stem guards - which protect the batsman's neck area - flying. He faced two more balls on the third evening, and a further ten balls on Monday morning.
A medical examination confirmed a concussion, and match referee David Boon subsequently approved West Indies' request for Blackwood to come into the game as a substitute.
A CWI spokesperson said Bravo had undergone a concussion test after the end of the third day's play, and passed it.
"He obviously got hit yesterday evening, but there weren't many balls left in the day's play, so he got through that and came off and he thought he was doing alright. He didn't show any signs of being dizzy or concussed," West Indies captain Jason Holder later said. "But he came this morning, and I don't know if it was because of the heat or what, you know, he was feeling a little bit dizzy, he had a headache, so we took the precautionary route and we pulled him off."
In what was to become the first instance of 12 batsmen playing in a Test innings, Blackwood came into bat at the fall of the fourth wicket, and scored 38 off 72 balls. West Indies, chasing 468 for an unlikely win, eventually slumped to a 257-run defeat.
Blackwood wasn't in West Indies' original squad, but is from Jamaica, and had earlier fielded as a substitute during the match. The last of his 27 previous Test appearances came in Bulawayo in October-November 2017. He becomes the second concussion substitute in men's Test cricket, after the Australia batsman Marnus Labuschagne, who came in for Steven Smith in the Lord's Ashes Test last month.
As with Bravo in this Test, Smith passed a concussion test on the day he suffered the blow, but experienced delayed symptoms the next morning. A Cricket Australia spokesperson had said then that 30% of all concussions in Australian cricket are delayed.