You can not pigment something that is White, black.
You have to pigment something that is fairly clear.
The glass beads are not optically clear- they have a frosted white appearance- like salt. Which is why the rigid is colored white- to hide the glass fill.
Ergo- even if you put them in a resin that is clear- the resin will look milky and never achieve anything but a grey color.
( and be careful about overpigmenting- it can dramatically weaken the resin polymer if there is too much pigment )
Self color is nice- but you can make rigid parts black with a $4 can of acrylic black spray paint and about 8 minutes of your time.
Rigid isn’t the strongest material they offer- its just one of the most dimensionally stable because the fill reduces warpage by reducing shrinkage-
as far as I can tell the strongest material they make is the Draft- that stuff is practically bullet proof.
For a really black rigid- they would have to fill the resin with a black colored material. Black resin already has an issue with laser penetration- a black filler might exacerbate that problem.