You can edit the texture for the visual effect. The easiest way to figure out which ffx is the blood would be to get the DSR debug menu, and turn on the option to show ffx IDs next to spawned ffx in game. Then you should find an ffx in common.ffxbnd with that ID. Unpack the ffx file with witchy to turn it into an xml, then fin the texture ID that the ffx refers to. You can edit that in the ffbnd resource folder and then repack.
#Blood visual effect
rotund nexus
Or just unpack common.ffxbnd and then go through the textures in it until you find the one that looks like blood, if you think that's easier