I've done both quite a few times now over the years. Your substrate isnt going to buffer your pH so you can drop that as a consideration. The amount of light reflected from white vs black is minimal at best and is likely no factor as well. This leaves you largely with personal preference but there are issues with some black sands. A local reefer used CaribSea Tahitian black sand and was having issues with elevated nickel and vanadium. Another local reefer took some to work and did some neutron activation analysis; he estimated the sand contained 300ppm vanadium.
I also use black sand but have Nature's Ocean Bio-active black and it's been trouble free (four ICPs done with nothing outside the norm in 2.5 years). These are the only two I can provide direct feedback on but be cautious with your selection of black. Oh, in regard to being magnetic, there are some pieces that will stick to a magnet in the Nature's Ocean sand but I've had none migrate to my algae scrubber (I have a blade so I dont get super close to the sand either). I do enjoy the look of the sand and largely chose it because the shrimp gobys are generally light colors and I wanted them to stand out better. Grain size is large enough to not blow around like crazy and fine enough that diamond gobys can still sift it.
I also use black sand but have Nature's Ocean Bio-active black and it's been trouble free (four ICPs done with nothing outside the norm in 2.5 years). These are the only two I can provide direct feedback on but be cautious with your selection of black. Oh, in regard to being magnetic, there are some pieces that will stick to a magnet in the Nature's Ocean sand but I've had none migrate to my algae scrubber (I have a blade so I dont get super close to the sand either). I do enjoy the look of the sand and largely chose it because the shrimp gobys are generally light colors and I wanted them to stand out better. Grain size is large enough to not blow around like crazy and fine enough that diamond gobys can still sift it.

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