you and Blaise dated for, 2 years and you were currently both in 6th year. (the year when Slughorn came and it was the Christmas dinner) you both went to this Dinner Slughorn decided to place on Christmas eve, and after taking your seats Everyone started talking

Then Slughorn asked her a question "So, how come you tend to keep away from other people?"

You had anxiety, and as soon as you felt everyone's eyes on you, you started figeting with the material of your dress and Blaise noticed and placed his hand on her thigh and gently stroked it with his thumb