I'm inclined to believe it. I recently watched The Aviator again, and Blanchett absolutely was Katharine Hepburn, bringing warmth and subtlety to a role that could easily have slipped into caricature.
If Blanchett could pull that off, playing someone so familiar to so many people, I have no doubt that she could inhabit the role of Veronica Guerin, or for that matter, anyone. Hell, if she decided to play me, I wouldn't give odds that my dad would hear the difference on the phone.
She's good alright!
She was also magnificent as Elizabeth I.