I typically use Légère Signature and American cut. Like both, but would love to include an even darker synthetic reed in my setup (not the Classic or Studio Cut 'cause I have a feeling those will eventually be discontinued).
I'm not sure why you are lumping the Signature and the American Cut together, other than the fact that both are made by Legere, since the latter is noticeably brighter IMO. Thus, "darker than the Sig" and "darker than the AC" are two different quests. Let's try the first one.
As noted by others here, the Forestone Black Bamboo is a good choice. (Stick with the regular Black Bamboo, not the weird Black Bamboo W Blast.) The White Bamboo is a little brighter -- more on par with the Signature. Forestone reeds have thin tips and tend to run soft, which some players don't like. The BB is a bit on the sturdier side.
The original D'Addario Venn was much darker than the Venn 2.0. It also easily eclipses the Signature. However, even assuming you can find some leftovers, you might not like its reduced playability.
The Silverstein Alta Ambipoly Classic (I can't vouch for the Jazz model) is pretty dark, but it's quite expensive, and also quirky because you have to wet it and treat it more like cane.
Some synth reed companies, such as Harry Hartmann and BARI, don't make a single model that would qualify as acceptably dark from a classical, blue box perspective.
The Classic is Legere's darkest reed by a wide margin, so it's a shame that it doesn't interest you. It's not as responsive as the Sig, however.
Not exactly a cornucopia of choices, is it? It doesn't help that the darker synth reeds often have responsiveness/resistance issues (Legere Classic, original Venn, Alta to an extent). Most synth reeds do tend to get darker as you move up in strength, like cane reeds, but of course that's a trade-off.
