UberBLACK was actually the very first car service that Uber had to offer. UberBLACK is essentially the luxury version of today's UberX. With this ride option, you can arrive at your destination in style with a black-on-black luxury sedan (or SUV if you're lucky).
UberSELECT is an option in the Uber app for a ride that's nicer than your average sedan. You'll be picked up by a highly rated driver at a price that's affordable for everyday use. You'll be picked up by a highly rated driver at a price that's affordable for everyday use.
UberX is NOT a Taxi. Depending on which city, it varies between an unregulated service to a highly regulated for hire service. Generally before Uber this same service could be had by calling a local 'mini-cab' or 'car-service' firm and booking. Neither is an entirely new service, just a more convenient and quality controlled service than before.