You could but performance wouldn't be that great; the 2012 models only have Thunderbolt 1 which isn't ideal for eGPUs. I suppose if you're not using it for gaming you might be alright.
In any case I wouldn't recommend the Blackmagic eGPU, mainly because it's overpriced and you can't upgrade it afterwards. If you've already got a video card laying around, I'd recommend just getting an upgradeable eGPU box instead.
EDIT: Come to think of it, does Apple even officially support eGPUs through TB 1 or 2?