SeaChem Onyx sand and grey coast calcite are the same thing, iron rich CaCO3.

The Black flourite is already available from most any SeaChem distributor............ask them to order it for you.

There is a slightly larger normal sized Flourite that's all black and then the finer onyx sand grain size.

The Plain black flourite is slightly smaller than flourite regular.

Also, you can easily simply siphon out that CaCO3 sand, or dolomite, and replace with silica sand.

No need to mess anything up.

Aesthetic sand should be a thin shallow layer also, sloped or use a hedge/wood/rock etc to lift up the planted sections with sediment.

regards,
Tom Barr