Black is the color of mourning in Western culture as well; most brides tend to avoid it, but that doesn’t stop Western grooms from wearing black tuxedos (possibly because the white ones make them look like waiters).
I’ve never heard of black sarees/lenghas/shalwar-kameezes/etc… being bad luck for Desi brides; it was my understanding that the brides avoid it because it’s so somber and not even remotely festive. Also, it’s so far from what are considered the traditional bridal colors of cream/ivory, gold, red and green.
For what it’s worth: I have nothing against brides wearing black, but I think black sherwanis are a bit much; they make it appear as if the groom is attending a funeral, not his own wedding.