I tried going to my sound effect dictionary for this one but nothing came up, any help in figuring out what that bolded bit of the sentence means is much appreciated.
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ジワリ is an onomatopoeia of liquid (blood here) oozing out / coming out (of wound, eyes etc.) It can also describe liquid (slowly) soaking (into cloth/fabric). Its variants include: じわじわ, じんわり, じわっ.
These are close to ジワリ:
じわっ Liquids soaking and oozing
じんわり Soaking slowly with sweat or tears
Quite a few onomatopoeiae have the same or similar pattern of variants, eg: