Helena (in A Midsummer Night's Dream) is saying that true love is able to ignore superficial beauty and appreciate the genuine value of a person. Cupid, being the agent of love, is therefore blind to appearance. Cupid is often portrayed in art as wearing a blindfold (i.e., "painted blind").
Her words are ironic and wishful, as she is lamenting Demetrius's infatuation with Hermia, and wishing his attentions were directed toward her instead.