About 1 pint (480 mL) of blood is taken when you donate. It takes about 10 minutes. The whole process-including answering questions and having a short exam-takes up to an hour. Donated blood is tested to make sure that it is safe to use.
Soda is an unhealthy beverage because it's high in calories and sugar but low in vitamins and minerals. Coca-Cola is one example of a type of soda that's sugary and packed with calories, but Coke Zero is one alternative that is sugar-free.
If it is indeed not harmful to humans at all - under this ridiculous amount that no one is going to drink - then I can only think it is a great supplementation to trying to lose weight. The water and caffeine in Diet Coke could only be of benefit.