In temperatures reaching 44 degrees Celsius (111 Fahrenheit), Brathwaite, who scored 94 in his team's innings and 212-run loss to Australia in the first test at Hobart, faced 178 balls before retiring at tea.
Blackwood was trapped lbw by legspinner Jeremy Hart, who took 4-93.
Wicketkeeper Denesh Ramdin, 38 not out, and Jomel Warrican, on 22, will resume batting Sunday.
Darren Bravo, who had a century at Hobart, was rested from the match in an otherwise full-strength West Indies lineup.
The second test begins Dec. 26 in Melbourne.
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