The Blue-Eyed Lass is a courtesan at the House of the Night in the Trade Quarter. If you pay her 500 orens (or 300 if you have a House of the Night Signet Ring) or give her a Sapphire, then she'll agree to sleep with you. If you attempt to bargain with her, then the price will go up, but if you exit the conversation and talk to her again, she'll reduce the price back down to 500.

Note: You can get a House of the Night Signet Ring from Erkyn Blunt in the New Narakort Inn. You can buy a Sapphire from a merchant in the Marketplace for 60 orens.

Another Note: You'll have to sleep with the Blue-Eyed Lass to complete the quest Blue Eyes. But you can also sleep with her outside of the quest if you want.