Wired "like" a dual transformer
This is a guess, but some thermostats require power to be delivered to Rc and Rh.  They will usually come with a jumper wire to bridge the Rc and Rh terminals, but maybe this installer just got into the habit of using two wires because that thermostat didn't come with a jumper or it was lost, or that's just how he liked it.

There should be no reason that both wires are used, and you should be able to use the black wire for the C wire.  Blue is typically used, but your thermostat wire doesn't have a white so your colors aren't going to match no matter what you do.