Well... hypothetically, yeah. Dark[ness] is, in practice, simply a school of magic, though one that's difficult to use.

Could you manifest darkness into physical objects? Hypothetically, yeah - it just depends on the density of aether and how long you can sustain it. Since the power used by Dark Knights is purely internal, it would only last a few seconds at best but... hypothetically.

We do know darkness can be used to bind - Igeyorhm does so after we defeat Bismarck so Thordan can get the key to Azys Lla. The way Dark Knights use it is almost purely for offense though, so that's another hypothetical.