I just had a great run ruined by saying to myself "the blood bag HAS to spawn THIS time!" and spending about 10 hearts + soul hearts worth of donations and getting no blood bag. 20-ish donations and no bag. I usually see it spawn in less than 10 tries, but apparently there is no set amount you can spend and be sure to get one, so be warned!

Anybody want to guess on the approximate odds on a bag per donation based on their experiences so far?

i was watching my roommate play... and he got it on his first go. i thought he said something about it being around 8-10 attempts usually.

Can you get more then 1 bloodbag in a playthrough if you encounter more then 1 bloodbank? I think i remember doing this but I'm not sure, so if anyone knows for certain if you can/can not then it would be appreciated! Hate to spend my hearts more then I should

@takayamasama: Yes, you absolutely can get more than one blood bag. I went one game where I was able to get a blood bag from every heart donation machine I encountered (3 in arcades and one from temperance). Of course, there is still the element of chance, and I too have gone 15-20 donations with still no blood bag at times.