Fun fact: it actually started in BWII with Black Lio Convoy who was Lio Convoy's clone in show and continued with Black Big Convoy in BW Neo. While of course neither of those is an Optimus Prime proper they WERE the first to pull that trend. Car Robots Black Convoy COULD be seen as the first to pull that move from an actual Optimus but on the flip side the way he debuted had him alongside a Convoy in a totally different mold. Armada's Nemesis Prime was the first time a proper Optimus Prime had a dark lookalike in the same body (with Takara's version being actually black). There's also, of course the black redeco of Takara's reissue of Convoy in 2000. So it's all a matter of how you look at it. I personally tend to credit Armada's Nemesis simply because Scourge/Black Convoy has remained a single character who's redecos are all specifically G2 or G2 based molds of Optimus whereas since Armada near every other Optimus has had a Nemesis Prime of some sort (and without Scourge's trademark teal more often than not).