aRE bLACK sNAILS jUST bLACK aPPLE sNAILS?

I'm not sure what you mean by "black snails". There are dark colored apple snails, but that doesn't mean every dark snail is an apple snail. If the snail is a pretty rounded shape, has a "door" it can close up on the bottom of its shell and has an air syphon (tube like thing it sticks out of its shell, often near the surface) then it's an apple snail.

You have probably just seem a very dark apple or mystery snail.

I've had apple snails that are very dark in coloring but I also have Japanese SNails that are black. Apple snails all have a syphone, any other snail does not.