'harmless' drone blades are the exception, if they even exist. Note that drones designed to operate indoors have some form of protection for the blades (Parrot drone, for instance).
In addition, 'cheap' blades have sharp edges due to the manufacturing process. Some drone owners recommend sanding them to get rid of those, to minimize injuries.
I could think of a pretty much harmless blade design that MIGHT work (soft blades), but they'd be woefully inefficient AND they would only work for those super-ultra-light micro/nano drones.
Anything else is not harmless. Even a tiny quadcopter can easily cut your skin, because the plastic is hard and sharp. Think of it as one of those plastic picnic knives, except not that sharp --- BUT rotating very, very fast.
In addition, 'cheap' blades have sharp edges due to the manufacturing process. Some drone owners recommend sanding them to get rid of those, to minimize injuries.