Moved to Shop Talk.
You probably "Glazed" the surface. It got very hot and the surface colors changed ( called oxide colors). The heat melted some of the belt grit and made a super thin glassy surface. It can sometimes be pretty hard and take a little grinding to remove. This is usually caused by the belt running too fast and you using too much pressure while grinding. No damage on the blade if done before HT. If this happened after HT, the blade would need to be re-done or scrapped.