At this rate I don't think Larian will ever fix this unfortunately, but I have read positive things about this mod: Clean Living

From what I understand, it should be available on all platforms and will permanently keep all your characters from accumulating dirt and blood. When adding to an existing save, you should have a Long Rest, have every individual character walk in water for a bit, or use a sponge/soap item to get the initial grime off, from then on everyone is supposed to stay clean. I've seen some people mentioning that a minimal amount of blood unfortunately does stay on Minthara for them, as well as a minimal amount of dirt on Jaheira, so getting that off might be impossible to achieve with mods.