Versatile winger Blair arrives at Sixfields having made 20 appearances, scoring three goals, in all competitions for the Cod Army this term following his summer switch from York.
Defender McGinty, meanwhile, has managed just three outings for the Blades this season, the last of which came in early September.
The 20-year-old, who can play at either centre-back or left-back, signed for the South Yorkshire club from Manchester United in the summer following loan spells with Morecambe, Oxford, Carlisle and Tranmere.
The pair are both available to make their Cobblers debuts at home to York on Saturday as caretaker manager Andy King looks to guide the club off the foot of League Two.
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