Some WS2811 are intended to work with 5V, some are 12V, so that depends on the particular LEDs you have. Some WS2811 LED strips have 3 RGB LEDs in a chain per “pixel” and those will definitely need 12V to work.
You can drive 12V LEDs with Pixelblaze by using a separate 5V power supply (or a buck / drop-down converter) for Pixelblaze. If you do this, be sure to keep GND connected between the 5V and 12V system. The data line needs that GND reference to work right.