yea, you can really see the dirt with the white sand. the black sand is just so contrasting with the bettas colors. You can see the dirt on the black, but it shouldn't be as bad as white...but w/ sand, it's eaiser to spot the gunk and siphon it out.
in gravel or marbles, the waste will sink to the bottom.
that's why for my tanks i like to keep them bare bottom...just makes easier cleaning for my 100% water changes.
in my 30 gallon i have those white flat marbles, and I hate them because i have algae growing on them and it makes the tnak look like dirty. I would go w/ black, but i'm too lazy to switch them out.