Getting 100% blood-bags is a big goal for players, but it's entirely random.
It would be good to have some a reasonably hard way of increasing blood quality of a prisoner or refine blood vials into higher quality:
1) Blood Transfusion - when you have a charmed human, you can interact with a prison cell and click "blood transfusion" - it kills your charmed human and gives prisoner some misery, but if charmed human is the same blood time and his blood quality is not lower than prisoner's by 10-20%, prisoner's blood quality increases. This way you can get 100% blood-bags by transfusing someone 60-90% several times with some other 60 to 70% blood-bags, but then you end up with a lot of misery, meaning extra fish. Could be made into separate station, like a cell for two prisoners.
2) Blood Refinement - would require a separate station, where you load two vials of blood and some herbs and it would refine them into one vial of higher quality blood.