... it's a UV diode with a phosphor on it, not a blue diode.
Nope. Just look at the spectrum of some white LEDs, they clearly peak around 450 nm plus what the phosphor delivers. UV is very problematic as it quickly degrades the plastic optics which are predominantly used with LEDs. Plus, you would only get the yellow light from the phosphor, not white light. It's the mixture of blue and yellow that's necessary where the ratio determines the correlated color temperature.