Surely the NWOBHM gave rise to a LOT of black/satanic/devil type bands, if you go just by lyrics and artwork? Iron Maiden, THE number one propenent of the NWOBHM, came up with the album "The Number of the Beast". Then there was Black Sabbath, Venom, Demon, Mercyful Fate ... the list goes on.
I THINK what you mean, maybe, is that in recent times the MOOD of HM has gotten very doomy, gloomy and bishroomie (sorry!)
, with a lot of bands focussing on the darker areas of life, but then the metal we knew in our youth (well, I did in mine) has now fragmented into so many different sub-genres --- doom, death, thrash, power, black, viking... again, the list goes on.
There are a lot of really good Heavy Metal bands still making "upbeat" music, like Dragonforce, Hammerfall etc, and of course Maiden are still around. I wouldn't say it's ALL dark...