In my story, one of my main characters has ice powers. She can turn water into ice, she can conjure ice (sucking moisture out of air and freezing it). She can also drop someone's body temperature, as she'd killed a whole lot of people with the great tool of hypothermia. But would I be taking it too far if I allowed her to be able to freeze the blood in someone's body, therefore killing them?
Its your magic system, you can decide exactly what you think it should be. Depending on your world, it may be taking it too far, or it may be a natural step. We cannot decide, only you can.
I would, however, highly recommend reading up on Sanderson's Laws of Magic. He's got a few laws that are very helpful for making a believable magic system, such as his first law:
Sanderson’s First Law of Magics: An author’s ability to solve conflict with magic is DIRECTLY PROPORTIONAL to how well the reader understands said magic.
If your main character has the ability to freeze someone, and she does so to resolve conflict, you will need to make certain the reader has a solid understanding of your magic.
Blood is able to be frozen, and at a surprisingly high temperature. -.6 celcius and/or 31 fahrenheit. However, the time it takes to freeze is determined by how fast the body temperature would drop to the numbers mentioned above, if you're going with that. You could say only the blood freezes and the body temperature does not drop, that would result in the blood starting to heat up, but by that time the person would die. (If in non-magic circumstances, that is.)