Harris was notoriously at accepting advice/criticism that contradicted his prior beliefs; many other times he was contradicted (eg on the utility of area bombing, on specific targets, on the use of his bombers by Coastal Command, etc) he reacted poorly- ignoring reports, stonewalling the other services or his superiors, and generally refusing to change Bomber Command policies. This would be doubly true in this case because Blackett worked for the Admiralty, and was disagreeing with a previous Bomber Command study. It's trebly true because he and Blackett had clashed repeatedly during the war- Blackett (and Henry Tizard) had (correctly) argued that Bomber Command's policy of area bombing was extremely ineffective, and some of its resources should be shifted to the other services, which Harris strongly disagreed with.