>>719191421
King d-mind is meant to be dark taranzas idea of what the most "savage" individual is, his "evil minds gather in his body" and create dark mind in the image of dedede, Kumazaki stated that in an interview.

The black mirror is found in "another dimension" in kirby clash so it has something to do with interdimensional travel like most things, idk what the significance of being able to cross dimensions is other than literally the ability to go across dimensions.

Hell galacta knight opens one up in his attacks in robobot and kirby clash that shoots a laser, you can even see the lor starcutter in it.