The Blood Bank in Old Blood is a place to store your blood where it cannot be stolen via losing fights.
There is a 10% deposit fee, so it is advised to deposit one less than your desired amount (1,000 nets you 900, while 999 nets you 900).

Withdraw: Lets one withdraw the specified blood from one's bank (only up to the amount of blood one actually have in one's bank).
Withdraw All: Lets one withdraw all the blood one has on hand immediately into the bank.  
Deposit: Lets one deposit the specified amount blood into one's bank (only up to the amount of blood one actually has on hand).
Deposit All: Lets one deposit all the blood one has on hand immediately into the bank.

Thanks to Robert Williamson for the heads up!