Just found this thread and Yes, I have said the same thing. I didn't realise anyone else thought the same though!
To my eyes, it started around 73/74, (although you could even say from 1969 things got darker somehow). I first noticed it with album covers. The hippy/free love/psychedlic era had pretty much gone and bands like Black Sabbath were starting out. Dennis Wheatley was very popular as were the Hammer Horror films. The Manson family were still very much in people's minds too.
As for actual 'darkness' in the literal sense, it certainly was for me growing up in dark, freezing old cottage, and a lot of the tv shows were as well.
People wore beige and brown a lot (as were the cars) and a heck of a lot of shows like the Avengers/the Saint etc had a lot of nighttime shots (ironically these were usually filmed in the day, but with a filter over the camera which gave it an even stranger ambience). Even American shows (often filmed in sunny California) weren't as 'glamorous' as they are today. The interior shots on Columbo for eg are often dark (perhaps partly due to the age of the film through wear and tear and storage), but more likely due to the dark browns and reds that were so popular back then.
Then in UK at least, it was the Yorkshire Ripper, IRA Bombings and Northern Ireland on TV every night which didn't help matters.
Of course you did still have the 'brighter' stuff on tv but I definitely remember it as a dark time.