Flank Steak Sous Vide - Blood Spots?
I would think the red pockets are liquefied fat and/or water with red dye in them (and some myoglobin, which is what makes red meat red, but often dye is the bigger reason).

If it's steak from a supermarket, it's probably thoroughly dyed, so maybe some of the dye (or a lot of it!) stuck with the fat.  Blood wouldn't be bright red, it would be oxidized and a nice brown/gray (if it were even in the steak - almost no blood remains in a steak, it's not why they're red).  If you're used to grilling or similar methods, those would allow the fat (and water) to drip off, while SV won't: you're packing it in a bag and keeping the water and fat inside (that being the point of sous-vide cooking, after all).