Given it's success here and style, easy to mess that up.
It`s the underlying gameplay mechanics that matter here, not style. Japan has produced a number of dark fantasy/medieval themed RPGs over the years - From Software`s own King`s Field and Shadow Tower series, Square`s Vagrant Story, Nihon Falcom`s Xanadu Next, maybe a few others that escape my mind now, but despite their unusual(for Japanese games) art direction they are still Japanese games through and through.
And Demon`s Souls was actually quite successful in Japan too. The initial shipment was incredibly small - only 20,000 copies(From Software`s Shadow Tower: Abyss for the PS2 didn`t manage to sell even that, so I guess the expectations were low for Demon`s Souls), but after the first batch of Japanese gamers bought the game, the word of mouth quickly spread and it ended up selling 170,000 copies or so. It also hanged in the charts for quite a while after getting a budget re-release under "Playstation 3 The Best" label.