Cold water usually helps to get rid of blood, I don't know how balenciaga's take to water ( I have a black one that shows no stains with water, but could be cuz it's black). If water doesn't stain lighter colored balenciaga, I personally (this is just my opinion mind you) would try some cold water and see if you can put little drops of water on the blood stain and blot using a clean side of a towel each time, trying to get the water to pull the stain out of the bag and into the water and then blotting. If you feel adventurous or have a part of the bag you don't mind testing you might want to try some mild soap with cold water and see that it dries ok without any discoloration. If you use any soap to remove the stain just be sure its mild and that you get all of it out and don't leave any residue on the bag.
I recently spilled cranberry juice all over the bottom of my foley and corinna mid city tote (leather) and I literally had to dunk the whole bottom portion (mind you the stain was not fully dried) into cold water and squeezed the juice out and allowed it to diffuse into the cold water. The bag looks perfectly fine now. However, foley and corinna bags are considerably cheaper and the whole underside was about to be ruined so I had nothing to lose. Just a little tale to let you know that bags can come back from something that seems super terrible!
If you are faint of heart about taking matters into your own hands I understand, but get it to a leather professional ASAP, blood is kind of like a pen stain in that once it sets sometimes they are really tricky to get out!
Best of luck!