If you're going to layer substrate, IMO in most cases you might as well mix it together right from the start, since it will get mixed over time and this way you have a more "uniform" look LOL
I've never mixed Flourite black and Onyx sand together. The Flourite is definitely black, but Onyx sand is grey. I do have Onyx sand mixed with black ColorQuartz in my 10gal, and the look is fine with me...
The main reasons I mix substrates is either to cut costs (especially for big tanks), add something special to boost nutrient content (I do sometimes add a super thin layer of peat since my well water is liquid rock), or just b/c I have it on hand and want to use it up LOL