I think soot sprites first appeared in My Neighbor Totoro?
you know, I was thinking the same thing because weren't soot sprites the ones that were in that house the family was moving into (in the totoro movie)? I remembered 'cause the little girl caught one but when she looked at her hands, there wasn't anything except all that soot!! I thought she was so cute! Nonetheless, spirited away and totoro are both great movies from miyazaki!!!
I wish I thought of making those soot sprites myself; they're so cute!!