In A Canticle for Leibowitz, the Plains Nomads conserve water for their animals by men drinking animal blood and women and children drinking milk instead. However, since ingesting any decently sized amount of blood can be toxic for normal humans, would it be possible for the Nomad men to have evolved the ability to process blood?
They already have, in Kenya
The Maasai of Kenya drink blood and milk from cows. While it isn't the mainstay of their diet -- current understanding is that it is reserved for ceremonial occasions -- they've been doing this for a long time. It would not be too far of a stretch to adapt that in your world building as a more prominent source of sustenance.
FWIW, I saw a short film on this cultural practice in high school (about 40 years ago); in that film, it was explained that they typically mixed some salt in with the blood before ingestion. That additive may be a mitigating factor in your worry about toxicity.
It is likely that the author of Canticle for Leibowitz drew inspiration for that Plains Nomads' practice from the real life custom of the Maasai. The milk drinking practices of some humans (IIRC, the neolithic steppe people spread it across much of Eurasia1) would be a similar adaptation.
1 David Anthony: The Horse The Wheel and Language
Yes, it's possible.
Given a population that has at least some variation in blood tolerance, it's possible for that population to select for enhanced tolerance given a pressure to do so.
Lactose intolerance is actually the default form of humans and many other mammals. However, in cultures where it was important, the survival benefits of being able to cleanly digest more milk products led to lactose tolerance spreading throughout those populations. This would be analogous, in that the survival benefits of being able to drink blood (surviving better in low other-water-level conditions) would be sufficient that people with that mutation would prosper, spreading it to their children.
This is assuming that there is such a mutation giving enhanced tolerance for drinking blood (which I've never heard of but seems plausible), or that one developed at some point in the intervening period.
Is it possible for humans to evolve to be able to drink blood? Yes, there are animals capable of drinking blood. It's not implausible that humans given the right circumstances could evolve to have the ability to drink blood.
That said they will never adhere to 100% Hematophagy. This is for the simple reason that drinking blood has a lot of drawbacks. It's hard to store, especially in primitive conditions, blood is also not well rounded in terms of nutritions (it can have lots of one sort, but very little of another for example way too much iron).
Here's a little article about blood sucking and why it really sucks having to make use of it. Sucking blood isn't an easy life
This is probably more of a comment, but blood is not going to be a substitute for water as the OP wants. Nor is milk. It's a simple issue of concentrations.
We need water to make up for loss through evaporation and excretion of wastes in our blood. Drinking blood adds water, but also adds equal concentrations of the waste materials our bodies are trying to get rid of.
See answers to: How long can a person survive solely on his own raw excrement and urine?