The Blackfish card says: "you don't receive losses stem from House cards abilities, battle symbols and Battle Fate cards (translation from Polish, I completely don't know the English meanings: "nie otrzymujesz strat wynikających ze zdolności kart Rodów, symboli bitwy ani kart Losów Bitwy").
How about confrontation with Balon Greyjoy: "Printed strength of your opponent House card is reduced to 0." Is it a loss from which Blackfish protects or not? Maybe English text on Blackfish card is more understandable?
Blackfish does not protect you from Balon.
Blackfish only protects against casualties (loss of units) due to swords and skulls from House cards and 'Tides of Battle' cards (which are apparently called 'Battle Fate' cards in Polish).