What happens when you "Offer Blood" in exchange for items?

"Offer Blood" means you gain corruption in exchange for the item. The corruption you gain is listed below the item, right next to the gold (circled in the below figure).

If the corruption you'd gain would result in you having 100 corruption, then you'll gain a new curse (when you leave the room), your corruption will be reset to 0, and then any excess will be added back. So in the below situation I'd gain a curse, and then my new corruption value would be 50.

It's also worth noting that it appears you can still buy items with blood even if you already have five curses (the maximum). I'm not 100% sure on this, but I was still able to purchase an expensive item with blood even though I was only 20 corruption away from unlocking the final curse. This just maxed my corruption out at 100, and unlocked the final curse (see below). I didn't have a chance to purchase anything else with blood before the run was over, so I can't be 100% certain of this, but it seems very likely. If true, then it would appear there's no downside to purchasing items with blood once you're already at five curses.