Well, Earth Blood reliably heals 10 Wounds in WFRP, while Healing of Hyish is only 1d10 and would cost us an extra favor. Admittedly the latter can be used mid-combat even without having someone cover us while we take a minute or so to use the item, so it's got some things going for it, but if each is 1/day or similar then we'd get a lot more out of Earth Blood and it's cheaper in terms of absolute wounds healed.
The bit about worked stone negating the spell is a good point- I was just thinking "underground, we'd be surrounded by earth" and may have made a significant mistake.
@BoneyM, comment on whether Earth Blood would be usable in a dwarfhold, please?
Boon of Hyish has a lot going for it and was my original selection of healing spell. With Earth Blood's reliability brought into doubt and favors no longer being assigned to Second Portent of Amul, since no one has actually spoken out against getting it- only in favor of other options- I wouldn't be opposed to swapping to it. Being able to cure others and cure poison in a matter of seconds as well as fix all wounds is a very potent ability well worth the five favors, in my opinion.
Other voters have opinions to share on this?