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This spell can't be countered.
Protection from blue and from black (This creature can't be blocked, targeted, dealt damage, or enchanted by anything blue or black.)
Although the reminder text doesn’t say so, Great Sable Stag also can’t be equipped by blue or black Equipment.
Great Sable Stag can be targeted by spells that try to counter it (such as Cancel). Those spells will resolve, but the part of their effect that would counter Great Sable Stag won’t do anything. Any other effects those spells have will work as normal.
