I think she'll like the Blast. I am really leaning toward getting one for my wife, who's about the same size. We have her on an M6000 now, and it's just too big for her to ride well.
As for a Blast in the mountains, I think with your wife on it, it'll be fine. If you plan to put a 200lb guy on it and then keep up with your kid who rides a turbo M8, you're probably not going to like it...
My first mountain sled was a 570 Mountain Cat. It had a 136x1.4 track, about 65hp at sea level, and weighed about 460lbs dry. I took it a LOT of places you wouldn't think it would go, but I was about 150lbs at the time, and my buddies were 200lbs on 800's with 144ish tracks. It kept up with them pretty well, and I had a lot of fun learning to ride. For comparison, the Blast is ~50lbs lighter, same hp but probably makes better use of it with EFI, and has more lug and more track. Plus Alpha.
Get one! If you don't like it, I'll come buy it from you next year!