Are you certain it's black algae? We have on occasion used Yellow Out chlorine enhancer to remover stubborn green and black algae with great success.

You probably are getting algae a bit more than usual as the plaster finish is getting rougher allowing for the algae to grow. New plaster will not "seal" it off. When the pool is prepped for new plaster, it should be thoroughly cleaned and it will remove the dirt and algae.

Water chemistry must be maintained upon filling and thereafter.

Improper water balance can lead to unfavorable consequences to plaster finishes as well as equipment.