Defender Rehman signed from Fulham this week, with striker Blackstock joining from Southampton, and the pair are expected to feature in boss Gary Waddock's starting line-up.
Waddock also hopes to have forwards Kevin Gallen and Marc Nygaard back for the visit of Steve Tilson's men.
Both players missed the opening two matches with hamstring injuries, but could recover in time.
However, forward Paul Furlong (thigh) and goalkeeper Simon Royce (groin) are unavailable, while defender Dominic Shimmin completes a three-match ban.
Southend are likely to be unchanged for the third successive game.
Manager Steve Tilson fielded the same XI which beat Stoke on the opening day at Crystal Palace in midweek and was rewarded with another promising perfomance despite falling to a 3-1 defeat.
Freddy Eastwood grabbed his second goal of the season at Selhurst Park and will again partner Lee Bradbury in attack.
Michael Ricketts (Achilles) and Efe Sodje (leg) are still missing.
Sporting Life.