Author of Pixelblaze here :)
FastLED is an awesome LED driver library with colorspace and fast math functions. I highly recommend it.
Pixelblaze is that plus a web interface, compiler, IDE, and pattern library all over WiFi. You can reprogram your work remotely without cables, and all real-time, or even on a phone. I found that quickly iterating I could get a better intuitive sense of the changes I was making and get the look I was going for with less hassle.
For apa102/sk9822 LEDs, the 32-bit math engine in Pixelblaze along with the HDR driver (5 + 8 bits) lets you get better color control at lower brightness levels and smoother transitions than what's currently in FastLED. I have a number of desk and/or nightlight projects that look just as good at 2% brightness, where with 8-bit per channel color you start to get extreme posterization and color mixes start failing.
Another slight advantage is that Pixelblaze uses a pipelined driver where pixel data doesn't usually need to be buffered before being send out to the LEDs. WS2812 are particularly sensitive, so there is a buffered mode for them, but for other chipsets Pixelblaze renders the pixel data just in time and doesn't require a framebuffer.