She is mortal. Remember, she had her hands burned by Drogon. Her immunity to fire is up in the air as of ADWD.
That isn't what I meant. Remember the LOTR books when that girl kills Sauron's guy? He said he couldn't be killed by a man, and he was right; he got killed by a woman. Same holds true here. The horn can't be blown by a mortal man (the one guy who blew it for Euron Greyjoy died because he was burned from the inside out). Strictly speaking, Daenerys can blow the horn and live because she is a mortal woman.