So...why doesn't Taxi specify exactly what their labels want and specify that they do not want old blues tunes... Because they don't know what a client wants until they ask for it. A client may well want an original blues music score or song to be used in a documentary or film or jingle, but they won't know that until they get a client needing it.
I don't think you really get how Taxi works. I don't use it because my stuff isn't commercial, but like Ronan said, they do exactly what they say they do.
And frankly, anyone who signs up for any service without investigating what they sell and to whom, and what the track record of success is, deserves to be separated from their money, because they're too stupid to hang onto it and use it where it would do more good. With all the info available at your fingertips, most musicians these days who get hosed are victims of their own laziness and ignorance. I've never been in a business where more people jumping into it have less of a clue as to how it works. There is no excuse in this day and age to be uninformed.