Could anyone suggest some way, via relay (I realize that would only be on/off) or mosfet, or PWM dimmer - which would to controlled via the PixelBlaze, to then control the White LEDs via their separate 24v circuit?
While I don’t have a specific circuit to recommend, I did want to mention that I’ve used the Mean Well HLG series of 24V LED drivers (get the AB suffix for dimming).
You can directly connect for a simple on/off control using digitalWrite(pin)
Also, a hack that popped into my mind was to hot glue a photocell against one of your RGB LEDs, and use that one LED as your dimming signal for the white channel! The “3-in-1” dimmer in that power supply means it accepts PWM, 0-10V analog, or variable resistance as a control signal (a photocell is a variable resistor whose resistance varies with the amount of light sensed). A little clever experimental level scaling in code means you might be able to dim the 24V white using the brightness of the 1 LED.