I didn't research so forgive my ignorance
It gets this property from its fine surface structure, which is a forest of tubes. Incoming light has to be reflected many times before it gets back out, so a black material is effectively made even less reflective. It's the optical-scale version of the pointed absorbers used in anechoic chambers.
It probably is not going to retain its blackness when exposed to water, dirt, or wear. Superhydrophobic coatings such as Never Wet have the same problem - they work because they're composed of tiny points, so droplets of liquid don't have a surface they can grab. But after some wear, the effect stops working. (See any of the many "NeverWet fails" videos on YouTube.)
This is likely to be great for protected environments, such as inside optical systems. It should be useful for optical sensors in space, too. But it's probably an inherently fragile surface. That limits its uses. (The "stronger than steel" probably refers to the individual carbon nanotubes, not the bulk material.)
This s a problem with a lot of surface chemistry stuff touted as "nanomaterials". They have interesting surface properties, but the surfaces are fragile, because they're some very thin surface layer with an unusual structure. If you protect that structure with some coating, you lose the effect.