As for the narration of Ali ibn Ja’far in particular, it is possible that the blood on the garment was the blood of a fish, since it is permissible to pray in both a small or large amount of it. And if a person has two garments, and one of them is contaminated with an impurity that renders it impermissible to pray in, then he should pray in each of them (one prayer in each), to be on the safe side. What indicates this is what was narrated:
Commentary #29