Looking at the back of my copy of Twilight Princess on Wii, it says it's rated T for fantasy violence and animated blood.
Really, if you think of how the ESRB works, T for Teen is really the only rating that the game could have gotten. Any game that features a realistic weapon can't be rated lower. So the fact that Link runs around with a sword means that the game will be rated T. The only Zelda games rated E are the cartoony ones where the weapons look cartoony and the games that came out before the standards changed in 2004. Under the new ESRB standards the ones released 1998-2004 would also be rated T.
It was actually games like Zelda that created the new ESRB ratings standards that started in 2004. Before the choices were eC (early childhood learning games), E (Everyone), T (Teen), M (Mature), and AO (Adults Only). Games like Zelda which featured violence and fantasy themes and were aimed at older kids were lumped in with arcade-style games like Mario and Sonic. An RPG problem solving riddle game doesn't have the same audience or playability of an arcade game, so the labels became very deceptive unintentionally. But the only choice was the unrestricted E or the restricted 13+ T genre. Because the content wasn't in need of sales restriction because it was generically non-offensive just simply geared to people further along than your average 10 year old, they labled it E. But in 2004 they started using E10+ and removed the restricted status of T. By using the E10+ games like Mario and Sonic could be divided from games that may need a little restriction not for content but for age group that the game is aimed at, such as the Lego series and Spore and the Sim City series games, which don't have content that's offensive just the game is too complex for little kids under 10. It also allowed games like Zelda and like the music games which feature minimally offensive stuff but are aimed at older kids because of the complexity of the games to go safely to T without restrictions on who could buy it. So just because something is rated T doesn't mean that the content is offensive, it just means that the game is at a level that somebody who's not a teenager would may be unable to play the game. It's like when you see a toy marked suitable for 10 and up. It's not that the toy is offensive or has content that is bad, it's just that it's not designed for people under 10 and doesn't have them or their needs in mind. I mean all of the Guitar Hero/Band Hero/Rockband etc. games are rated T for teen. The swears are bleeped out and all of the songs are radio-ready (meaning you can hear them on the radio), there's no violence, no gore, no sex, it's just that the game is too hard for the little kid set.
Also remembering the ESRB is entirely voluntary and when games are submitted for review by the ESRB it is submitted along with documentation from the developer on what age group they are aiming for with the game and what rating they developed the game with the intention of seeking. That speeds up the processes of the ratings system where many, many games get held back and launch dates moved (L4D for example and I believe Dante's Inferno as well). It's entirely likely and probable that Nintendo submitted TP saying they believed it should be rated T because that's the audience they were aiming for and the ESRB agreed and issued it.
And when you see what the ESRB standards for "animated blood" and "fantasy violence" are you see that the rating is the most benign rating in the world:
Animated Blood - Discolored and/or unrealistic depictions of blood
Fantasy Violence - Violent actions of a fantasy nature, involving human or non-human characters in situations easily distinguishable from real life
I think that describes Zelda, all Zelda games, released to date. Though as I remember the blood in TP was, I thought, spirits and not blood. But it's been years since I played it.