what do you think camus is saying by having daru struggle as he does, only to find the message on the blackboard?

There is irony here that reflects the irony that Daru has always known about his existence in Algeria. Despite his affection for the people, he will never be considered one of them. Daru gives the prisoner a choice of where to go, freedom or prison yet he is condemned as a French oppressor who handed an Arab brother to the French authorities.