The area looks practically normal but the edges are sore and the skin in the center is numb. Is this a kind of mild frostbite?

Frostbite can have. blister or red or can turn black. In all it is a thermal injury much like a burn with tissue destruction and the deeper the extent of the frostbite, the deeper the injury ( much like a burn) the more destruction there is. Swelling and reddened skin is rather mild, followed by blistering, and then on to loss of circuation at deeper and deeper levels.If u have a mild case do not re-expose to cold .