Hello,
i would also prefer black! Hm ... i use black gravel in my big tank (where my lap, delhezis, pollis aund teugelsi are living). In the case of delhezi i can say that the colouration and the contrasts are boosted in a enormous way! After a few days i could see a fascinating difference ...
Some bichirs love the slower way - my teugelsi needed time and now his colouration is more vivid than any time before ... the black patterns are REALLY black!
In my smaller tank i have nearly white gravel (where my senegalus and weeksi are living). I think senegalus isn't that wonder of colouration (as every pic i saw told to me), weeksi is somewhat "strange" in colouration - the most weeksi i saw had amore greyish colouration on the back, mine is more brown/orange ... maybe a variation independent from the gravel.
Another fact: Bichirs really love the twilight, the black gravel helps dimming the light in a very natural way. For my opinion the bichirs seem to feel safe with the black gravel (also with the plants) - maybe a reason why my bichirs are growing very well.
Greetings
Uwe