There's a difference between what you said and how you said it.
You could have walked up to the cashier, then quietly and politely said "Excuse me, there's a blind woman over there waiting for help. Would you be able to get someone to help her?"
Tellling her that she ought to be ashamed of herself when, as Mindy said, you have no idea what she, or the other members of staff, can or cannot do, was out of order.