It is fine, however, you want to keep it on a low setting. Also fyi, heat dehydrates your muscles so be careful using an electric blanket for periods longer than 20-30 mins. Id also recommend talking with your obstetrician about this.
I've been using an electric blanket for about a month, I keep it pretty low temperature and I always end up taking it off and on etc
I haven't heard that it's unsafe though obviously you wouldn't want to crank it up or anything. I'm also a pretty small person so I always have need of extra heat. I don't imagine you'd be having it on you if you were uncomfortably warm.
Maybe give your doctor or nurse line a call and make sure?
I don't believe that it is bc it makes your body super warm which is not good for the fetus. When I pulled my back out with this pregnancy my nurse told me that I could only use a heating pad on my back for no longer than 15 minute spurts.