Its because of her ice magic --I'm assuming here-- that allows her to withstand certain levels of heat. So to that exent; I myself, as a fanfic writer, personally see Elsa with a blue enchanted heart; thusly allowing her to use ice magic. Its better than saying her blood vessels were swimming with ice shards. Then again her blood vessels swimming with ice had been used before in another fanfic {Title The Ice Princess over on fanfiction.net}
This is just me saying this really, my views are mine and mine alone, I do not speak for others. The curse that was put on Marian is to kill her by freezing her heart yes, but she was not struck in the heart. Hans and Anna, and also Kristoff were frozen into statues yes, because they weren't hit in the hearts. {which killed Helga {sniffle}}
Now, on that note; when does a heart become enchanted to begin with ? I know that Emma's heart is enchanted due to being born of true love.
And if anyone wonders why I think Elsa has a blue enchanted heart, Well, I think it was true love between her parents to give her an enchanted heart, but her ice magic to give her a blue enchanted heart, a living frozen heart.
An interesting though I will admit, but its just me thinking this. I personally like the idea of such a thing. Now if only I can get the right screenshot of same heart and edit it to blue