Vampires and Soylent Green?
If vampires were real, could a person live on blood alone? Also, in the film Soylent Green they dried people out to make tablets for other people to eat. Is that feasible?
Yes. In theory you could do that. Blood is actually full of protein and it's full of iron, which you need in your diet. Don't forget that black pudding is just congealed blood that you fry. Blood also contains water which you need and there are some salts in there, so you can get protein, salt and iron from drinking blood.
There will be other things that you can't get from blood like water-soluble vitamins which are not present in large amounts. There will also be some fat-soluble vitamins which don't appear in large amounts in blood. You can probably make a reasonable meal out of blood but it wouldn't make up for all of your dietary requirements and so you might want to eat some freeze dried body pellets to supplement your intake. That's no different, really, from eating a microwave meal which is effectively just a dried out bit of body. It may not be human, it's just animal instead.