Can I use an at-will ability to fuel an ability that is fueled by spending a use of that ability?
For example, dragonfire adepts can use any invocation they know at-will, one of which is
You can create magical darkness, as the darkness spell (PH 216).
(Dragon Magic, p. 79) On the other half of the combo, there is the feat Blend into Shadows (Drow of the Underdark, p. 47)
BLEND INTO SHADOWS
You can draw from nearby magical shadow to cloak yourself in darkness.
Prerequisite: Darkness as a spell-like ability.
Benefit: As a swift action, you can spend a use of your darkness spell-like ability to make a Hide check, even while being observed and without cover or concealment. You must be within 10 feet of an area of magical darkness.
You also gain one additional daily use of your darkness spell-like ability.
So, can I use the Darkness invocation to activate the benefit of the Blend into Shadows feat?
Consider the following: According to D&Dtools, there are three feats with "per day" (my guess as to the relevant words) in the prerequisites. These are
- Bonus Breath ("[b]reath weapon with limited uses per day", Savage Species p.31),
- Improved Web ("[a]bility to create webs as an extraordinary ability at least twice per day", Savage Species p.36), and
- Extra Exhalation ("breath weapon with limited uses per day", Races of the Dragon p.102).
The existence of these feats implies that the BiS prereq. could have been worded as "Darkness as a spell-like ability [with limited uses per day]".
Furthermore, Savage Species was published in 2003, Races of the Dragon was published in 2006, and Drow of the Underdark (DotU) was published in 2007. Therefore, the precedent had long since been set before DotU.