Actually in some productions, Fantine was a brunette. Besides, hair color doesn't matter. Her hair can be beautiful, luscious and desirable without it being blonde, even in those days. Seriously, this really doesn't matter at all. In the original London and Broadway productions, Fantine was a blonde. Both Patti LuPone and Randy Graff were costumed with blonde wings. It wasn't until much later that they started color-blind casting, but in the original production Fantine was conceived as Hugo described her -- blonde, reason being that her golden hair was her greatest asset. Fantine is blonde for a reason. Her hair color is plot relevant and mentioned a lot in the book. Throughout her section, descriptions of Fantine all emphasize her beautiful blonde hair. It's worth so much when she sells it because she's blonde more so than because it's long. The musical does not depart from the novel sufficiently to obliterate Hugo's symbolism and rationale for making Fantine blonde. Fantine's blonde hair represented, to some extent, purity and innocence. When she cuts it, she loses that. The symbolism just isn't the same when the character blends in with everyone around her.
Salve, Regina, Mater misericordiae
Vita, dulcedo, et spes nostra
Salve, Salve Regina
Ad te clamamus exsules filii Eva
Ad te suspiramus, gementes et flentes
O clemens O pia