I feel your pain! My son could not blow his nose until he was 8 years old, and he also has allergies, a sensitive gag reflex and vomiting from the secretions. Not what you wanted to hear I'm sure, but I can tell you what the occupational therapist & pediatrician told me to do to help him learn it. One suggestion was to have him in the bathtub blow bubbles with his nose underwater. Since he couldn't swim underwater we didn't do that one, I didn't want him inhaling when he should be blowing! Practicing in the tub can help though. Another was to have them blow bubbles with a regular jar of bubbles. Apparently, nose blowing takes a lot of muscle use and coordination, and so does blowing bubbles, so they somehow are related. If she masters blowing bubles, then maybe she is ready to blow her nose. Also, tell her you are going to block her mouth and have her breathe out of her nose so she can get the sensation. Lastly, tell her to blow real hard, like she was blowing out candles with her nose. Practice, patience, and more patience. I think you will have to wait until she is ready for it to work. My son missed a lot of school bcasue he was always wiping his nose with his hands and I found that to be gross!
We made a big deal out of it when he finally could do it. Good luck!