You could get black trim on a Sandman in HX and the first half of HZ, but it was Slate Black (18V) rather than Black (19V). The difference is the seat and door trim material. Black is single tone, Slate black is a two tone vinyl so it looks "off-black". It was the case from HK onwards where Belmont got single tone vinyl, Kingswood got 2 tone. This was exactly the same deal for the other trims as well, i'll give you an early HZ trim for example (HX is too difficult as there are bench seat codes to deal with). It just shows up more easily in black as they get two different trim codes ie 19V and 18V.

Example: 67V. In this trim code HZ Holden and HZ Kingswood share the same code but it has a different name (like Black and Slate Black) for the two luxury levels. In Holden 67V is brown. In Kingswood it is Tan. Like the blacks, the difference is in the vinyl on the seats and door trims. It just is more obvious on the blacks as the trim code is different. Same thing happens in HJ where 60V is Beige on Holden and Belmont and Chamois on Kingswood.

Note that 18V being the only black on Sandman only applies in HX and HZ. In HJ Sandman got the same trim as the base vehicle, so on HJ Holden Sandman you got 19V if you ordered black trim, but on HJ Kingswood Sandman you got 18V.