You'd use it like Honey. At 5 to 15%, it lightens body and adds alcohol. But, it has a pretty light flavor, most of the time.
I'm familiar with golden to deep amber Agave nectars, but not "dark." If that's a term for a deep amber, then you're probably cool. Super light colored versions can have added sugars (though fructose is the main one from the plant).
I think it's an ingredient that needs a counterpoint flavor to bring it out. The best Agave beers I've tasted back it up with a little toasted malt, like honey malt, Maris Otter, Amber Malt and even Victory. One other had added depth from Oak chips.