Yes, that is the same thread.
To Paul....No, there is no such subspecies, or phase of a Mexican Black king known as a "Desert phase", in California kingsnakes there is, but NOT MBK's. It can only be a Desert king(L.g.splendida), Mexican Black king(L.g.nigrita), or an intergrade composite of whatever percentage of the two as I posted earlier.
Now going by phenotype alone, as it is right now at approx. 21 inches as you stated before, I would simply go with it being a darker than some Desert king(L.g.splendida) unless it gets VERY black later on, in which case it would then be considered a Mexican Black king(L.g.nigrita). If it stays basically just as it is, it is definitely a Desert kingsnake(L.g.splendida) all day long.
This is a good Desert kingsnake example I own, but they can also be much drabber and less defined too and still be very genuine Desert kings.

Here is another one I own, and is a actually a sibling to the above one as a matter of fact. Notice there is substantial variation between the two.

And this is an excellent Mexican Black king phenotype I used to have, but "SOMETIMES" they can have an identical splendida pattern when VERY YOUNG, and gradually become a solid black snake(MBK). Most in the hobby today are much more solid black as hatchlings, and this is only because it is a more "preferred" characteristic to most folks, so the blacker one's are always the one's that are selectively bred in today's hobby. But in the wilds of the Sonoran Desert in Mexico where importation and study is impossible to very dificult, there are quite a substantial number of them that hatch out "looking" like splendida(Desert kings) but are NOT. And then they undergo a gradual ontogenetic change to solid black just as my example shows.
Now considering they are 21 inches now, and weren't fed very well before you acquired them, I would honestly say they are straight-up Desert kings at this point. You really cannot get a more accurate answer than this.
Hope this helped you undestand this a bit better. Things in nature are not nearly as cut-and-dry as alot of other things...
~Doug


~Doug
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"a snake in the grass is a GOOD thing" 

serpentinespecialties.webs.com