Was the builder working to any plans / specifications. Depends on what blocks he has used, do you know? The fact that its not seen is no reason to use potentially inferior materials, that potentially will have a shorter lifespan than brick, plus its far quicker to to lay blocks rather than bricks. I really wouldn't worry about upsetting the builder, he hasn't got to live with it long term. Best advice I can give you is that you get a surveyor to have a look and see what he thinks. Hopefully you are paying by stage payments.