Black sand is not advisable with Corydoras, because they do not "expect" this and they become stressed by having to darken their pigmentation in order to blend in, which is their primary defense reaction--remain motionless and blend in with the substrate and the overhead predator will not see me! It is no coincidence that the basic body tone of so many cory species is buff/taupe...it is exactly the same tone as the sand these species live above. There is no other issue other than the above that I am aware of, and this would have much less impact on upper fish like guppies (I think white would be more likely to stress upper fish because it reflects light unnaturally).
Having said that, to your questions. Do you just have one cory? This is a very social fish and it needs a group around 10 (or more), so this needs to be thought through. As for changing the sand, if it is smooth fine sand I would probably leave it. You can add hardscape to break up the sand, chunks of wood, smooth rocks for example. Leave some open sand as this is where cories feed.
I would never mix different sands, this in my view never looks good, it is certainly unnatural, and it visually draws attention to the small space. The uniform substrate whatever it is will do the opposite, make the space seem larger. And, if the two types are sized differently, the smaller will drop to the bottom with the larger on top.
