pigmentosa most commonly affects young children.
The good news is that childhood urticaria pigmentosa usually, but not always, resolves by adolescence.
Apart from antihistamine medicines, there are certain specialized treatment available like Ultraviolet light treatment, Potent steroid
creams & Laser
Each of these modes has pros & cons which must be taken into consideration.
Consider discussing these treatment modalities with your skin doctor & leave him to decide.
Otherwise the only way to remove those black spots immediately is by using camouflage make-up.
Hope you find the information useful.