Depends on how well the ceiling fan was installed, sometimes it's just into the ceiling drywall or plaster, sometimes it's into a thin piece of wood between 2x4s sometimes into a 2x4 directly. Even then it might be mounted with just a couple of tiny wood screws, or with big bolts, it's really random how well they are attached. When they are installed they only have to hold 10-20 static pounds, they are not designed to hold up a person, or the huge dynamic load of a falling person, and even installed properly they aren't designed for that kind of load.
If it's built well and it is mounted well into a solid piece of wood I'd imagine a rope going around all of the blade roots would be strong enough, I wouldn't have the rope around a single blade root, that's just asking for trouble if it wasn't secured enough or if it is loose.
I wouldn't trust ANY ceiling fan by just looking at it, you have to know if it'll hold any amount of weight by inspecting it or actually testing it. Preferably with more weight and more fall distance then you'll actually need for the real thing. If you are unable to actually test it or know for sure it'll hold I'd look for another method or location, it's not worth the risk that it'll fail and I'd consider it unreliable as a hanging attachment point.
