At the close on the opening day of the third and final Test, West Indies were 276 for six, with Blackwood unbeaten on 45 and Jason Holder not out on five.
Ramdin perished for 53 about half-hour before the close, after putting on 94 for the sixth wicket with Blackwood.
The partnership was critical after the tourists, perched on 172 for four at tea, lost veteran left-hander Shiv Chanderpaul without adding to his tea time score of nine, to the first ball after the break.
However, Ramdin struck six fours off 103 balls in 113 minutes at the crease while Blackwood has so far counted five fours off 102 deliveries, as they denied the South African attack any major success in the final session at Newlands.
Debutant off-spinner Colin Harmer has been the best bowler with three for 67 while fast bowler Dale Steyn has taken two for 59.
Earlier, Leon Johnson struck his second Test half-century but Marlon Samuels missed out on one, as West Indies suffered key setbacks during the second session.
The left-handed Johnson, unbeaten on 24 at lunch with West Indies on 80 for two, scored 54 before falling lbw to Harmer about 50 minutes after the resumption.
He faced 84 balls in 109 minutes and struck nine fours, adding 51 with Samuels who made 43 before gifting his wicket, 15 minutes before tea.
Looking at ease against the South African attack, Samuels gave Stiaan van Zyl his first wicket in Tests when he lazily drove the first ball of the part-time medium pacer’s first over to Faf du Plessis at cover.
Samuels faced 70 deliveries in 109 minutes and counted six fours.
Johnson had earlier added 50 for the second wicket with opener Devon Smith who made 47 before falling on the stroke of lunch, bowled by Harmer.
Smith and Kraigg Brathwaite, who made just seven, put on 30 for the first wicket after West Indies won the toss and opted to bat, making good headway against the hosts’ new ball attack of Steyn and seamer Vernon Philander.
The left-handed Smith started positively, driving Steyn to the cover boundary and pulling him square for another four, in the fifth over of the morning.
Brathwaite cut Philander to the point boundary in the day’s tenth over but departed four overs later when he edged Steyn to Dean Elgar at gully as he pushed hard off the back foot.
Smith and Johnson then impressed with an enterprising stand. Johnson got off the mark with a pull for four off Steyn but it was Smith who took centre stage, punching Steyn sweetly past mid-off and following up in the next over from Morkel with another attractive drive to the cover boundary.
He was in sight of his sixth Test half-century when he attempted to cut one too close to him off the last ball before lunch, and was bowled by Hamer.
Smith hit eight fours off 86 balls in two hours at the crease.
Johnson, who had gathered two boundaries off Hamer’s third over prior to lunch, made good progress after the break in tandem with Samuels.
He pulled Steyn for four and repeated the shot against Philander to move into the 40s. When Harmer was re-introduced, he greeted him with a drive to the cover ropes, to raise his half-century.
Johnson was growing in confidence when he was trapped lbw to Harmer at 131 for three and Samuels added 31 with a sedate Chanderpaul before falling with a half-century looming.
When Chanderpaul was stumped down the leg-side off the first ball after tea, West Indies had lost three wickets for 41 runs and in danger of the now familiar collapse.
However, Ramdin and Blackwood repelled the Proteas attack, to haul West Indies back into the game. The right-handed Ramdin got off the mark with a streaky boundary to third man off Harmer before taking two successive boundaries off Morkel behind square on the off side.
He was unbeaten on 38 at the final drinks break and boundaries off Harmer and Philander took him to 49 before a couple to cover saw him to 12th Test half-century.
Disappointingly, he fell four overs later, getting a leading edge back to Steyn off the bowler’s second ball of a new spell.
The 23-year-old Blackwood, on seven at tea, survived a difficult chance to wicketkeeper AB de Villiers off Harmer on 13 in the fifth over after the break, but made the most of the opportunity.