Here in the printer’s manuscript, Oliver Cowdery initially wrote “in the skirts of his cloak”. Later, with slightly heavier ink flow, Oliver crossed out the in and supralinearly inserted upon. This change could be due to editing, especially since the use of in here seems so strange. Of course, the correction could have been made when Oliver later proofed 𝓟 against 𝓞. This is the only example of the prepositions in and upon being mixed up in the history of the text, although there are a few examples of mix-ups between in and on (see the discussion under Alma 50:15).
Elsewhere in the text, blood always “comes upon” someone or their clothing, although the reference in these other cases is to responsibility rather than to actual blood:
In any case, the text consistently supports the use of the preposition upon when referring to blood. This evidence supports the occurrence of upon as the preposition in the original manuscript for Helaman 9:31.
Summary: Accept Oliver Cowdery’s correction in 𝓟 of in to upon in Helaman 9:31; this correction probably occurred when Oliver proofed 𝓟 against 𝓞 (no longer extant here).