My guess would be that the hair mesh has blank vertex colour data, which is used for some color blending in the hair shader. If the data is blank it will cause the entire hair material to be set to the vertex colour parameter (which is likely to be black if it hasn't been set).
Vertex colours should default to white, which would cause no additional blending, but I've seen some meshes where they are set to black.
Try setting the "Vertex Color Strength" parameter to 0.0 in all the hair materials for that mesh, from the material parameters panel:
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