Question
Why Gandhari remained blind-folded all her life
Answer
Gandhari has blindfolded her eyes because of love and respect towards her husband Dhritarashtra. I don't think there is a role or impact of Bhishma on this incident.
It is mentioned in Sambhava Parva of Adi Parva, Mahabharata.
Soon after Bhishma heard from the Brahmanas that Gandhari, the amiable daughter of Suvala, having worshipped Hara (Siva) had obtained from the deity the boon that she should have a century of sons. Bhishma, the grandfather of the Kurus, having heard this, sent messengers unto the king of Gandhara. King Suvala at first hesitated on account of the blindness of the bridegroom, but taking into consideration the blood of the Kurus, their fame and behaviour, he gave his virtuous daughter unto Dhritarashtra and the chaste Gandhari hearing that Dhritarashtra was blind and that her parents had consented to marry her to him, from love and respect for her future husband, blindfolded her own eyes. Sakuni, the son of Suvala, bringing unto the Kurus his sister endued with youth and beauty, formally gave her away unto Dhritarashtra.
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