A little girl wrote a poem to him because she was at the school he was at when he visited it, and it mentioned his blue eyes. His eyes aren't a vivid blue. They're... a sort of piercing blue with specks of grey, which is possibly why they're so... compelling to look at.
And omg omg omg my historical excitement from this photo just CAME. [link]
All those original pictures are never close up enough to clearly determine his eye colour. If you take a picture, with a modern camera, of someone with blue eyes from a distance, it looks dark because it's not close enough. PLUS, all eye-witnesses say they remember his shockingly blue eyes.