Blower motor maybe, but code B1421?

Two different things. You have some problem with your blower and no code for it (that's not shocking, even modern cars with obd still can have things go wrong that don't set codes), and, unrelated to that, you have B1421.

B1421 isn't quite any 'trouble' code really, just a tool for you to know whether the dash solar sensor works. If you have B1421 in the dark or the shade, and you don't have it in bright sun, the sensor works.

 

I'm pretty sure if you take it the glove box and then peer in through the recirc air inlet, or pop out the cabin filter and peer in that slot, you can see if the blower is turning.

If it isn't (and you have things set so it should be), you may also be able to give it a little nudge (watch your fingers) and see if that starts it.

 

I think maybe giving it a nudge with the car completely turned off, to see if it spins OK, would be the place to start. If it won't go around by itself for a turn or two after a push the lubrication on the bearing is gone or there is something in there jamming it up. The latter can be fixed, but the former is pretty much a death sentence for the blower. If there are leaves or twigs in there it will often make a sort of periodic crunching noise while it tries to turn. If it spins easily then pull off the external wiring and test the internal wiring with an ohmmeter, there might be a short or open. The next step is usually to see if applying 12V to it will make it run - but only if the blower is designed for that, some expect PWM or other inputs. I don't know what the Prius blower wants.

Little computer DC fans can often be resuscitated for a while by lubricating them - ones that wouldn't start by themselves will once again. That often doesn't work with larger DC blowers, where access to the bearings to apply the lubricant may not even be possible. Example - the HV pack blower's plastic impeller is permanently mounted to the motor shaft. It is possible to get the case over the motor off the back, providing access to the back bearing, but I have never found a way to reach the front one because that impeller is in the way. If this blower's impeller can be removed, then maybe it can be lubricated.