Part of this matches with my head cannon. The biology is the same, though less of a difference between male and female. But the powers are rather different; they are able to form things out of their body, though it is heavily draining for them to do so. And instead of being shadow, they can hide well in it, but like how invisibility would just bend light around you, if you look close enough, you can see a faint outline.  
And in mine there is not as much of a gender difference, no gradient on the legs (at least in pure umbrum), the muzzles are angled for males and curved for females. females are usually shorter, have a thinner build, and are better at stealth because of that. Males are usually taller, have a broader build, and thus often prefer to go weapon on weapon. There are the males that do stealth and the females that do attacks, though.
 
Not saying it’s bad, just that we’ve had probably the same inspiration for this, and came up with different ideas.  
I find it interesting.
 
I’m glad that people made a reference sheet for umbrum.  
Do you have a language yet? If not, I’d be happy to help draft. I’ve got some notes you could use.