but he could still use dark firage and during the eterna session limiti he could use a barrage of dark firaga. Also dark shield.
And he still can though. As stated before, Riku can still use Dark Firaga in KH3. It's
incredibly strong, when analyzing the programmed values for it and is probably his best move when controlling him.
Not to mention that, while not explicitly stated that it is "darkness" element, he utilizes quite a few warp skills such as his dodge literally seeing him teleport via a little dark orb looking thing. Considering the fact that Vanitas (pure darkness) uses warping moves on the regular I tend to feel that it is a somewhat darkness aligned move.
I think it is important to note that in the KH universe there is a very important difference between using dark-attuned abilities and "using the darkness" (or "being used by it" as some might say). While a bit confusing, the first suggests that you are utilizing darkness in the same way you would use an element (ex. thunder surge or fire raid) whereas the latter is letting the darkness, or negative emotions, dictate what attack you are using.
Take the first sparring match between Terra and Aqua for example: Terra was sensing he was at a disadvantage/losing and either out of frustration or desperation, suddenly felt darkness appear in his hand. It startles him, and he corrects it, but it was clearly unintentional and troublesome. Riku, in his earlier days was letting his jealousy, frustration and rage and various things "power him up" which you can see most clearly in KH1. When you fight him in his full hula-skirt dark attire at Hollow Bastion entrance, his attack moves are actually nearly identical to those he was using on Destiny Islands...but they are powered up by these excessive blue/black laser beams of sorts. This is even before he gets "possessed" by Xehanort. Clearly we can see in this case Riku was "using the darkness" as a funnel for his emotions, not using it as an elemental upgrade to his ability.
This is hugely contrasted by how you see him fully actualized at the end of his arc. He has completely different "darkness" element moves that are incredibly powerful but more importantly under his control.
Riku in KH1 could use dark firaga (or something similar), but only seemed able to do so when very angry. Grown-up Riku can use it at will with incredible precision and ease, and doesn't need to be raging out to do it.
This is the difference, and why it was very important and valuable for Riku to come to terms with the darkness. If he continued to use darkness out of rage and anger it would eventually consume him until there was nothing left.
Honestly, if we take the whole "darkness" thing out of it, Riku's journey is actually about a teenage boy going through puberty and trying to navigate all the emotional changes that come with it. The game just added a little fantasy twist to it. Riku being able to understand, accept, and control his emotions are signs that he has matured and made it through that tumultuous period into a better actualized human being. This is what makes him stronger now.
