some armor parts (and weapons) can get + spell damage / - enemy lightning/fire/cold poison/ resistances , regardless of the shrine you use , reason why your roll had more than -8% before blessing ; most probably you were luck enough to get both the item affix and shrine affix to spawn at the same time . that's how you know if it's a good craft (if the blessing goes well also)
check this list https://docs.median-xl.com/doc/wiki/affixes if you want to know exactly how much each item can spawn with