Costelow won his first senior Welsh caps during the Autumn Nations Series, while Blacker was on the bench for one game during his first involvement with Wayne Pivac’s squad.
The Scarlets were beaten 36-19 by the reigning URC champions, the Stormers, in Cape Town last week and Dwayne Peel will be hoping his team can end a run of three successive defeats to notch only their second win of the campaign.
Tongan international Vaea Fifita is also available to play this weekend after missing out on the game in Cape Town.
All four Welsh regions are playing in South Africa this weekend in Round 9, with the Ospreys kicking things off on Friday night against the Sharks in Durban. Toby Both’s side picked up a try-bonus point in their 43-26 defeat to the Bulls last weekend, while Cardiff Rugby ran out 35-0 winners over the Sharks at Kings Park.
Cardiff head to Loftus Versfeld to face the Bulls on Saturday, while the Dragons face the Stormers earlier in the day.