I read that they were flame-retardant on a moving forum. The information is from 2004, which is why I mentioned that I don't know if this is still the case. I just called U-Haul myself, just out of curiosity, and they actually told me that they couldn't tell me for sure whether the balnkets are fire-retardant or not. So if anything, looking into it more is warranted (like looking on the label on the actual blanket, if there is one or maybe contacting the blanket supplier). For myself, I am going to stay on the safe side until I know otherwise. I think people should be aware it's a possiblity.
Actually, I just thought of this - there is a really easy way to check. Cut off a small strip of the blanket and (in a safe environment, preferably outdoors and having something to extinguish it) try to light it with a match. If it smolders rather than catches fire, the blanket contains fire retardants. I don't own these blankets, so I can't run the test myself.