Yakuza: Dead Souls has always been considered a spin-off though no? It's non-canon, I wouldn't consider it fair to call it a black sheep entry when it isn't considered mainline. I find 3 the weakest of the mainline but it's no black sheep, lots of people love it and it's still a great game.
Remember when, after Dead Souls flopped, Sega thought that the western popularity of the frachise was dead, and it took many years and a sony intervention to get them to release Yakuza 5 (in a digital format only)