The Japanese name for Night Slash literally translates to "Crossroad Killing," and refers to the samurai practice of testing new swords on innocent passerby, most often at night. The Pokédex entry for the move Dark Pulse states that
"The user releases a horrible aura imbued with dark thoughts." So it's about more than the energy that appears when the move is used. It's about the basis behind the move. As
@lolipiece said, Dark type Pokémon are meant to be tricksters who cheat and use stealth and the power of darkness to defeat opponents, and many Dark type moves reflect that.
What the Japanese name is for Night Slash means is totally irrelevant, because games/anime Night Slash doesn't involve any stealth and dirty tricks or cheating the opponents. Game/anime Night Slash is simply like: slashing the opponent straight up with dark/crimson coloured energy, which is representative of the Dark type. And so Night Slash is a Dark type move. There's no stealth involved there.
Similar with Dark Pulse, it involves dark/crimson coloured energy. A dark/crimson coloured burst of energy/beam is released at the opponent, that's what games/anime Dark Pulse basically is.
So as we can see, that Dark Pulse and Night Slash are Dark type moves because simply that they involve dark/crimson coloured energy, which is representative of the Dark typing, not because of any stealth, dirty tricks or cheating the opponent. So saying all Dark type moves has to involve dirty tricks, stealth or cheating the opponent is quite wrong TBH.
Yes, dirty tricks, stealth or cheating the opponent are also an important aspect of Dark typing, that's undeniable. Many Dark type moves like Feint Attack, Torment, Sucker Punch, Knock Off, Pursuit, etc. involve stealth, dirty tricks and cheating. But those aspects aren't definitely the entirety of Dark typing either. There are also other aspects of Dark typing/Dark type moves: like involving dark/energy, involving something nasty, brutal or ferocious, etc.
So saying that Bite and Crunch aren't that much Dark type, because they don't involve any stealth, dirty tricks or cheating, like what
@lolipiece was suggesting, doesn't make much sense. Stealth, dirty tricks and cheating are one important aspect of Dark typing but not the entirety of it. Bite and Crunch are Dark type moves likely moreso because biting and chomping down on someone bis supposed to be something nasty, brutal and ferocious.
Yep, Satomine Night and lolipiece are right. Writers are having problems with Ash's Pokemon learning any Dark-Type moves (other than Bite and Crunch).
Like Nanu said, Ash is not a "Dark" guy. Ash is supposed to be a friendly character, not dark character. Dark-Type is known as "Evil-Type" in Japan.
Scraggy and Greninja haven't learned any Dark-Type moves. Krookodile has learned Crunch, but it replaced this move with Aerial Ace.
Ash's Pokemon like Totodile, Krookodile and Lycanroc knew/know Bite and Crunch, which are Dark type moves. And that invalidates the entire point.
Just because Scraggy and Greninja didn't knew any Dark type moves, people tend to assume that Ash's Pokemon can't learn Dark type moves or so, because Dark type is known as the Evil type in Japan. But this notion is quite wrong, because if that was the case, Totodile, Krookodile and Lycanroc would have never had Bite/Crunch in the first place.
Krookodile's Crunch was later replaced, so what? It was replaced very late, in the League. Krookorok/Krookodile still had Crunch for a
long amount of time, and used it for a long amount of time. If learning Dark type moves were really an issue, then Krookorok/Krookodile wouldn't have known Crunch for a long amount of time in the first place. Just because it was replaced later, doesn't change the fact that Krookorok/Krookodile had and used Crunch for a long amount of time.
Dark-type is called Evil-type in Japan. From the old Eastern perspective, evil is more like dishonorable tactics. In an honor-based society like the ancient Chinese, Japanese, or any nation with an edition of Code Duelo, you fought your enemy face-to-face. Both men are armed equally and it becomes a matter of skill. You don't kick your enemy while he's down or stab them from behind.
Read above what I said to Satomine Night. Night Slash in the games/anime doesn't involve any stealth or anything like that. It straight up slashes the opponent with dark/crimson energy, that's it. So what the name of Night Slash means in Japanese means, is totally irrelevant, since the attack itself in games/anime doesn't involve any stealth whatsoever.