Dark TNG?
“Where Silence Has Lease”, “Q, Who”, “The Survivors”, “Family”, “The Wounded”, “The Drumhead”, “Violations”, “Man of the People”, “Frame of Mind”…
Off the top of my head.
"Where Silence Has Lease" (S2) was disturbing, but I wouldn't call it dark.
"Q Who?" (S2) had Q and the Borg, mysterious and intense, but I wouldn't call it dark.
"The Survivors" (S3) had a dark ending, but the overall episode is more in the WTF category.
"Family" (S4) is hardly a dark episode.

This is called "drama."
"The Wounded" (S4), this is just a drama about a man who can't move on from war.
"The Drumhead" (S4), since when are courtroom dramas "dark?"
"Violations" (S5), haven't seen this since I was a kid, so no comment.
"Man of the People" (S6), is this the old-Troi episode? I don't think I've ever seen this one.
"Frame of Mind" (S6), I'll give you this one, this is definitely the "Riker must suffer" episode.
Even "if" these were "all" dark episodes, that's what, one or two a year?
Season 1 was frequently dark, maybe not whole episodes, but dark scenes were frequent.
This was caried over into S2 a little bit, and then the show shift away from this in S3 onward toward drama over
space is dangerous (compared to S1-2 anyway).