Experienced midfielder Ravenhill, 32, arrives at Sixfields on a one-month deal from Bradford in search of regular first-team football having not started a single league match for the Bantams this campaign.
The former Doncaster and Notts County man has been granted permission to feature in Sunday's FA Cup clash with Bishop's Stortford.
Blyth, meanwhile, rejoins the Cobblers from Championship side Leicester on a loan deal that will keep him at the club until the middle of December.
The 21-year-old striker enjoyed eight appearances for Aidy Boothroyd's struggling side earlier this season, scoring three goals, before seeing his scheduled three-month stay cut short towards the end of September due to injury.
Blyth has also been given the green light for FA Cup action this weekend as Northampton look for a welcome boost having slumped to the foot of the League Two table.