Two highly-mobile badass females in a fight to the death. Who do you think would win in a Death Battle?
My money would be on Blake.
Blake is more maneuverable than Mikasa, who relies on her 3DMG to maneuver, and Mikasa's gear is vulnerable to mechanical problems and fuel shortages, meaning she's screwed once it runs out. Blake is also tougher than Mikasa, having taken an explosion at her feet from Torchwick's gun and getting up seconds later like it was nothing, while Mikasa once took a hard fall and was incapacitated from it. Blake's Aura also affords her more protection, so Mikasa loses out there.
Mikasa does possess the reflexes necessary to block a bullet in flight, but Blake has deflected automatic fire (as shown in the Black Trailer). Gambol Shroud might be a match for Mikasa's blades too, as it could cut through Atlesian Knights like they were paper. Mikasa's only real advantages would be physical strength and combat experience, but Mikasa's experience is against slow, lumbering Titans, not a fast Faunus like Blake.
Lastly, Blake's Semblance would make this fight totally unfair, as Mikasa would have zero idea on how to deal with it. If Blake combined Dust with her Semblance, it'd be a one-sided slaughter.
Even if the result is abundantly clear, it doesn't mean a fight between them wouldn't be awesome to watch.