This is a concentration upon nufs, the cause of all disharmony. When one concentrates upon God, nothing but love will be found, but when the attention is centered around the thought of self, all ugliness, pain and illness will rise. Of course in a certain sense they were always there, but this concentration gives them more life. It deprives the body and mind of the usual life which is naturally bestowed upon them by the Grace of God. It feeds the elementals who derive their potency from the excrescences of man, and these elementals in turn increase his trouble.
Concentration upon darkness does not increase the darkness, but it does impede the opportunity for light and health to reach the place of sickness.