According to ICN, 'Black Knight' doesn't exist; plant referred to as such is actually species Echeveria affinis itself. I don't think your plants are it, since E. affinis has narrower leaves. Your plants could be actually same - possibly a hybrid 'Black Prince' - I do not see much difference between two. They were probably in different location - one has bit of green on leaves. They loose dark coloring when light is less than it should be. It isn't that much green yet, so slowly introduce to good sunlight and it will be all dark color. Make sure it is gradual, or plant could sunburn. If/when it blooms, post photo - blooms will help to ID it. I am not 100% sure if it is 'Black Prince' since the leaves look quite plump (more than I usually see) - but it could be.

Btw, what 3rd are you thinking of - or you are just wondering what else it could be?

Thank you kindly! I didn't know that the lack of light could make this one it lose its coloring.

As for the 3rd, I wasn't sure if it could be something else entirely :) Thank you kindly!

Insufficient light will cause change of color in many different succlents. They will also etiolate - stretch too much, grow skinier and have leaves spaced apart or recurving. Plant can recover from change of color - if given sufficient light, color will correct. Recurved leaves will also correct (mostly), but if branches and/or stems etiolate, they will not 'shrink' back...

Plant above could be totally green in low light.

Here is 'Black Prince' - very low light on left (leaves are also skinny, and you can see how far apart they are on top, smaller plant); full sun on right: