The Radziner has the siyata that the cuttlefish is actually blue (when it feels like it) and it also undulates with the water, so it is domeh layam in two respects that the murex cant, though they are both mollusks, the cuttle fish is also, more “like a fish” than the snail.
On the down side, the blood or body fluid of the chilazon is supposded to be black,( or what the Rambam means by “shachor”) which neither the octopus or the snail present.
There are also some indications that both camps have the color wrong. If kale ilan is a reasonable look alike, than techeles should be the color of, pardon the expression, brand new, unwashed blue jeans.
For talis shekula techeles, you can’t make the strings black, because THEY WOULD LOOK LIKE TECHELES. (hilchos tzizis bet/chet)
The bottom line, is that while it is hard to figure what the Rambam means with the firmament reference, the plain havana is that the color is really dark blue, diluted from a black/blue fish juice, and the processing is only to make it color fast. A really dark purple might also fit, but not the milky bladder juice of the snail nor the sepia brown of the cuttlefish whose must color change in essence,through elaborate chemical processes.
In Medrash Tanchuma on Shlach, the techles is called ganuz. Not lost, but hidden, taken away for time unspecified. As we will not find the aron, the ketores, the other kelim, we will not find the blue juiced blue fish untill it is revealed in its right time, and we are restored to our destiny.