TV Tropes puts the start of darker and grittier more into the 1990s. See https://tvtropes.org/pmwiki/pmwiki.php/Main/DarkerAndEdgier

And some even in the 1980s or 1970s. https://tvtropes.org/pmwiki/pmwiki.php/DarkerAndEdgier/Batma...

I don't think the 2000s were especially darker and grittier?