Aidy Boothroyd promised changes after seeing his side fall to the foot of the Football League at the weekend and has moved to sign the pair, Leicester City striker Blyth for his second loan spell of the season at Sixfields.
Experienced midfielder Ravenhill arrives from Bradford City.
On-loan Peterborough midfielder Kane Ferdinand is also free to play in the cup tie while talks are ongoing with Brentford about the use of Stuart Dallas and Luke Norris.