Is it possible for buck boost converter to output -48V with +12V input?
Maybe you need to search for 'inverting buck boost'?

Think about what's going on if you have a grounded inductor. 'Charge it up' through a switch from the +ve supply. Then open the switch. Current is flowing into the inductor, so it continues to flow in, and drives the output -ve. As far -ve as you like. Job done, set the switching times to stabilise the voltage.