Krylon "Crystal Clear" works well...
If you get the satin finish, it will make it just about impossable to tell which blade was sprayed after you're done. (I'm thinking APC here...other props you might want a glossy finish?)

It also helps, from a durability standpoint, if you scuff the blade to be sprayed first with very fine (600 or finer ) sandpaper.
(I wipe the prop first with denatured alcohol or wash in warm/soapy water to be sure there isn't any mold release on it...)