I just looked, and you're correct - the XM16E1 lower in black is on backorder. Butt...I saw they have the XM16E1 available as a blem. They say: "However, these options have varying finishes, ranging from shiny green to dark gray."
I just got a blem M16A1 lower from them (picked it up from my FFL shop a couple days ago), and it's a dark gray (almost black). I'm going to use it with a black upper, and it looks very close.
It's a really good deal at $95, and you could afford to send it off for black anodizing if after you get it you think it's too light.
Below is my blem lower with the upper I'll put it with. It is lighter than the black upper it will be with, but it would be easy to get it anodized black - but I think I'll try to lighten the upper to match the lower!
ETA: pic added