I'm a sucker for the genre of Twilight Zone-type shows and with this being pretty much the current modern embodiment of such, I can't help but watch and enjoy. But yes, it is dark for the most part. I mean, it's right there in the name, "Black Mirror". (which, is that meant to refer to electronic screens? I've started to see that term being used.) There was one episode out of the six this season that I though wasn't particularly dark and actually kind of nice: Episode 4: Hang the DJ (and it does refer to The Smiths' song, no DJ is actually hung) which DBZ referred to above as being a possible metaphor for how 4D and above might interact with 3D. So if this show seems like the kind of thing you might not be able to stomach but you are still curious, maybe that one is worth a try.
The other thing that stands out to me about this show is that, with the possible exception of Season 4 Episode 4 above, the one thing all the episodes have in common is the technological materialism that sometimes veers towards transhumanist. There is not really any hint that a reality beyond our observable one exists unless it is somehow created by technology. As darkly as things are portrayed in most of the episodes, it does not appear to be advocating a recklessly pro-transhumanist worldview, but I wonder, are they maybe advocating some sort of more ethical, balanced, materialist pro-transhumanist worldview? As with any show like this, the interpretation is ultimately left up to the viewer, and it can certainly be seen as a satire of materialism, but are the creators of the show firmly rooted in materialist thought and promoting it in a warped way? It's not really clear.